Banana Phone Blocks Robocalls

Despite the implementation of the National Do Not Call Registry in the US (and similar programs in other countries), many robocallers still manage to get around the system. Whether they’re operating outside the law somehow (or they simply don’t care about it) there are some ways you can take action to keep these annoying calls from coming through. [Alex] is among those to take matters into his own hands and built a specialty robocall-blocking device.

Based on a Raspberry Pi, the “Banana Phone” is able to intercept incoming calls on standard land lines or VoIP phones. After playing a short message, the caller is asked to input a four-digit code. Once the code is correctly entered, the caller is presumed to be human, added to a whitelist, and then the Pi passes them on to the recipient. There are, however, some legitimate robocallers such as emergency services regarding natural disasters or utility companies regarding outages. For these there is a global whitelist that the Pi checks against and forwards these robocalls on to the recipient automatically.

This project was originally an entry into a contest that the Federal Trade Commission put on a few years ago for ideas about how to defend against robocalls. We covered it back then, but now there are full build instructions. Even though the contest is long over, the Banana Phone is still in active development so if you have a spare Pi lying around you can still set this up yourself. There are some other interesting ways to defend against robocalls as well, like including the “line disconnected” tone in your voicemail, for example.

48 thoughts on “Banana Phone Blocks Robocalls

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      Next up, a method to detect this Banana Phone is being used and bypass it automatically with voice recognition software.

  1. I did this over a decade ago, with asterisk and a dial in card… Complete with rude messages and a rabbit hole for people who wanted to type random numbers in and call forwarding to my mobile…

    1. Me too. I received exactly ONE “robocall” ever and I’m not even 100% sure it was a robocall. The guy was a salesman of some sort and he was curious about my system. I was amused and told him I did telecom work. I have a bit of a daunting message that sounds like the phone system screwed up and that you have to dial tech support. It scares off my m-i-l’s boyfriend from calling which is a good thing IMO.

  2. I only have my cell and i get the same robo calls from different numbers each time. i just don’t answer my phone anymore unless it’s a known number. the robocalls have never left a voicemail.

          1. Maybe open the article by noting that it is an update instead of your snarky response? I don’t blame the commentor, articles are sometimes recycled around here…

    1. Re Scott Vinzant: I suspect this isn’t true indignation but rather a reader trying to show off what an “insider” he is for knowing about a four-year-old article. Either way, it always amazes me that people are so unaware of how childish it looks that they’ll post comments like this one.

      On the other hand, snarking about the typos you’re clever enough to spot is a true class act.

  3. This is very similar to an idea I had some time ago. I wanted this for cell phones in particular. Assign each person their own 4-digit pin code. If you don’t have a pin and all me, you go straight to voice mail. Someone gets too clingy or bothersome, revoke their pin and they can’t call you anymore. This would have been particularly helpful in those years I was single but dating in my 30s. I’d too often find out why those gals were single, they were nuts!

  4. Sadly I get a constant stream of robocalls thanks to my curiosity one year finding out how bad it could be to not pay the identity obfuscation protection money to a crooked (but very old and definitely mainstream) domain registrar…
    The answer (in case you were curious) is that robocalling spammers scrape and resell numbers from the whois database and three or four years on I still get at least one business loan scam robocall a day…

    1. This seems like it would be great in the form of a smart phone app, but I don’t think the carrier’s generally let you mess with a non-rooted phone at that level.

          1. This!

            I do something similar. My cell phone has a 3 second silence MP3 as the default ringtone. Everyone in my address book has a distinct ringtone assigned. People NOT in my address book just don’t “ring”. This gets rid of robocalls and wrong numbers. Using it for over 10 years :D

    2. Unfortunately, I was told that .US domains don’t let you allow identity obfuscation for some reason, so I ended up putting in an incorrect phone number after I received a few robocalls. I’m sorry, for the robocalls you’re now getting, Sheraton reservation hotline.

    3. This makes me think you can make a good honeypot for robo callers with just a domain registration. Pretty inexpensive source. Publish that number only in that one place and see who calls it. you would know the source and that all calls are bunk.

  5. There is a free service that blocks robo calls. I have been using it for several months and it works 95+% of the time. The service is If a call actually gets through you can add their number to the websites directory of blocked calls and help block the caller for other subscribers. It works.

    1. We got the land line cut off when we moved to NBN (fiber to the home) and didn’t replace it with voip as, the call centre scam artists got a bit much.
      The political parties never rang the mobile (cell phone) cost them to much for “hi I’m doing a political surv *click*…”
      Just had a bout of insurance and survey scams over the last month
      on the mobile. But I think they finally have the message. If not….. I’ll get rude, or I’ll really waste their time… “sorry he is in the other room… I’ll go get him”. And leave it on hold… or fake a murder so they freak out.

      1. I have a landline connected but only for my TPG ADSL (for various reasons it makes more sense for me to stick with it than to switch to Naked).
        I get no unwanted calls on my mobile phone (occasionally I will get calls from businesses I already buy things from trying to sell me other things I dont want but its fairly easy to get them to go away)

      2. We’ve had the land line here long enough for it to have gained two digits: it used to be a 6-digit number.
        As for NBN, lucky you. I’m still waiting for one of Turnbull’s cronies to turn up with some string, two tin cans and a 300 baud acoustic coupler. (Which will seem fast compared to what we probably will get.)

  6. I prefer the paranoid approach: I never answer a call from any unknown number, unless it calls twice or three in a row. A robocall (or a telemarketing operator) never insists on a missed call as they have a long list of numbers to follow.

    1. That’s mainly what I do– rely on spider-sense and tell everyone who gets my number they better leave a VM because I won’t answer. Still need to update my VM greeting so it says once again “tell me why you’re calling” because the people who call or email or text just to say “call me” are more annoying than any spam calls. Like we all have time for that :P

  7. If you don’t what to go full blown *Asterick. The CID modem to raspberry route is open.

    One ringy dingy, and gone. Only problem you must be the smart one and blacklist
    the call. That process is painless. Client apps to handle that. (android and PC).

    Beware there are persistent callers like. “Feed the children and UWVC” In the case
    of feed the children. Even spring, they come up with 20 new numbers and call….

  8. I’m pretty sick of the amount of unsolicited calls that roll in, especially the scams. Maybe 30% of the calls are just silence.

    Fortunately, the majority of unwanted calls have a pause then an obvious connect to a ‘boiler room’, some have this big boing sound as they connect, making it easy to just hang up as soon as you hear that.

    But I’ve been thinking about something like this… good timing HaD.

  9. Caller ID is just plain broken. My landline phone gets 10 – 20 spam calls a day, from fake Microsoft support technicians, to extended car warranty providers. The ‘scripts’ they use are more advanced now. Ie, there is normally a pause as they listen for your voice (to start and stop quickly, like someone would saying – hello – rather than a long message like an answering machine). So, while you notice the pause, the recorded message says ‘oh my, I had a problem with my headset. There, much better. I can help you with a car warranty for your….’

    Any caller ID number can be spoofed. Typically, the scammers use just a city/state now, as it seems to allow for the best pick up (rather than – Microsoft Support or Jimmy’s Car Extended Warranty Service).

    The system needs to upgrade and provide a way to verify/sign that the caller is who they say they are (ie, public key cryptography).

    1. Just pick up and hangup. NEVER say hello! Newer systems are just trying to get a voice recording of you accepting a call. If they can get you to say “Yes” then they can use your voice to do stuff. I forget what that is. I saw it on the news recently.

  10. While I welcome any method to block those lifetime-killing illegal calls (illegal here in Europe), I don’t get how the “whitelist” is supposed to work.
    The caller ID presented to the phone can easily be hacked (and gets faked all the time around here), so whitelisting won’t help a bit, if the caller either does not publish a number at all (we tend to not accept calls without a displayed ID, although every once in a while we gamble and consider it might be a new client) or if the caller ID presents some official numbers.

    Whitelisting based on caller IDs is a 100% failure in my eyes.

  11. I believe it is my civic duty to keep the scammers on the line for as long as possible…

    “Hello, I am calling from Windows, Your computer has virus!”

    Then I just give them the run around, and play dumb, until they give up. ;)

  12. It’s time to eliminate the ability to change the caller ID phone number. I can think of no valid reason why this capability even exists. Once that happens then blacklisting is super-simple.

    1. It is eliminated for people with 1-800 numbers. I think they used to be called WATS lines. I’m not talking about 1800 redirect lines but actual landline that has a real 800 number (877, 888, etc.). They only show ANI numbers not CID which can be spoofed. ANI is what the phone company uses to do toll accounting so it can not be spoofed. That’s why you should never try to prank a 800 number or even a 911 operator as they will get your real number. Then the phone comp will give out your CN&A (customer name and address) without a warrant. You can just call 411 or ###-555-1212 and ask for a reverse listing on a phone number and they give you the address if it is not non-pub. does a good reverse number look up too.

      Caller ID spoofing was meant for legit people who need to disguise their true CID like Abused Women Shelters, Law Enforcement, and Private Detectives/Investigators. But people like SpoofCard ruined that idea.

  13. Most landline providers like COX cable in USA don’t really give you any good automated methods to block unwanted calls. I use Google Voice with a transfer to my celly. It does good blocking techniques but is not the most efficient. I found that the ringtone method is quick and dirty for low level cell phones that do not use iOS nor Android. There are many Google Play (et al) apps to do this but are not 100% accurate.

    The ringtone method mentioned above is quite slick. What you do if you can’t find a silence.mp3 ringtone is to download the kid’s mosquito.mp3 ringtone (20,000 Hz). It seems only teenagers or younger can hear it. Us old timers can’t. Just download it to your old dumbphone and set it as your default ringtone. Then setup a quasi whitelist of friends, families, and business associates in your contact list and give then unique ringtones each or just set them to that annoying old time phone ringer (your choice but it’s more readily available on dumbphones). The telemarketers, scammers, robocallers, and new friends that somehow got your celly all do not bother you with a ringtone you can hear. Who cares if it bugs the heck out of your kids?!

    Your missed-call message inbox does fill up. Just periodically do a DELETE ALL every week. I think you can do the same thing with the SMS text section too. For a really cool instant comm system with friends and family with smartphones is to use and it sounds like the old NEXTEL walkie-talkie system. I use it all the time and people ask me how’d you do that? I thought NEXTEL was gone! it is zello is like a cool replacement. You can send images and text messages too.

    I found some NEXTEL direct talk units on EBAY and they work great for like 4-5 miles simplex comm with digital FH spread spectrum. Very secure. Don’t need Internet for them to work like Zello. However, I found a walky-talky app that uses the ad hoc peer-to-peer network connection. Great for survivalist comms where wi-fi and internet is down. That’s like a 2.4 Ghz QRP rig. Then there’s BearTooth. Great application but is kinda’ expensive per unit.

  14. A few years back, I was getting junk faxes from an unknown number on a number of medical clinic fax lines. They were cruise ship BS things. BUT they did leave a call back number on the fax that traced to Florida, what’s a person to do with 3 PRI T1 lines each with 23 channels and only about a %15 call volume.
    Well put that spare capacity to use with Asterisk of course. I did a DOS on number called it with 60 calls at the same time. only 8 channels connected before I got busy tones.

    AHH you can figure the party only has a hunt group with 8 lines and the same few staff to answer.

    I politely asked for them to take the entire area code for the area I was at out of the fax spam. nope they didn’t want to. They asked for my number, I told them I have a number pool with over a 1000 lines in 3 cities and they didn’t want to remove them all and they weren’t going to avoid my area code.

    I was nice, at first

    But after getting the brick wall treatment. I took a more militant approach. I set asterisk up with an automated message seeking enforcement of my right not to be contacted for solicitation after being informed, and set it to dial the crap out of their listed number with 10 dedicated lines. I kept a log and let it record any call that was actually connected because well I found the foreign language swearing to be top notch!

    They let the phone ring for hours with no answer. Then the next day, started answering phones again, I felt it my mission in life to prevent these people from doing this again. After a week of denial calls they closed up shop.

    Then what crosses my fax system, another set of unsolicited faxes with a new number.

    Out came the old script, down went their lines. My phone company was amused and let me play with the ANI and CID info a bit as well so I could do things like place the call but have the Caller ID say – “Trouble”, “Give UP”, “Fax me again”, “Irritation” along with not very nice things. I also set the CID to their own number, and to my surprise I got their voicemail system! WHO the heck uses CID for voicemail authentication. Welp that led to even more avenues of amusement.

    In the end I did manage to make contact with the company one last time and they threatened me, I told them call the cops I would be happy to explain what went on and provide a recording of every call I made asking you to remove me and you ignored each automated message, all 60,000 or so of them.

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