Hush Those Old-Fashioned Phones

Most people hate unsolicited calls, and it’s worse in the dead of night when we’re all trying to sleep. Smartphones are easy to configure to block nuisance calls, but what if you need a solution for your Plain Old Telephone System (POTS)? [Molecular Descriptor] has built a system to invisibly stop landline phones ringing after hours.

The basic principle relies on an analog circuit that detects the AC ringing signal from the phone network, and then switches in an impedance to make the phone company think the phone has been picked up. The circuit is able to operate solely on the voltage from the phone line itself, thanks to the use of the LM2936 – a regulator with an ultra-low quiescent current. It’s important if you’re going to place a load on the phone line that it be as miniscule as possible, otherwise you’ll have phone company technicians snooping around your house in short order wondering what’s going on.

The aforementioned circuitry is just to block the phone line. To enable the system to only work at night, more sophistication was needed. An Arduino Mega was used to program an advanced RTC with two alarm outputs, and then disconnected. The RTC is then connected to a flip-flop which connects the blocking circuit only during the requisite “quiet” hours programmed by the Arduino. The RTC / flip-flop combination is an elegant way of allowing the circuit to remain solely powered by the phone line in use, as they use far less power when properly configured than a full-blown microcontroller.

It’s a cool project, with perhaps the only pitfall being that telecommunications companies aren’t always cool with hackers attaching their latest homebrewed creations to the network. Your mileage may vary. For more old-school telephony goodness, check out this home PBX rig.

26 thoughts on “Hush Those Old-Fashioned Phones

  1. Hum, this prevents the phone from ringing by picking up the call, this sounds like a terrible idea to me. Usually a collect call just needs you to stay in the line after a message to connect and start charging your line. Someone using one of those when a prankster calls them is going to have a nasty surprise when the phone bill arrives…

    1. He could simply have it “pick up” for just a second and then hang up. It might be easier to simply open the line (disconnect). It wouldn’t hang up on people, just keep anyone from answering. It would have less flexibility on placement than his (would have to be upstream from all phones if he has multiple), but it would work.

  2. When I was a kid, I found that bridging the two wires of the line would make the line show as busy. Now that I’m not a kid, I wonder how unholy that is to do to the phone system, but still — a relay that bridges the lines together at night would have the benefit of seeming busy rather than answered.

    1. This hack is really dependent on how the local telecommunications network is set up and what the local laws are. Can’t speak for other countries, but in Australia, you can be fined for plugging non-approved (by the ACMA) devices into the telecommunications network.

      This includes unapproved cordless phone base stations, or anything you might fabricate at home. Shorting the line will likely trip a circuit breaker in the exchange for your line, and that will have the engineers knocking.

      1. No, shorting the line won’t trip a circuit breaker, because subscriber loops are fed over a “hybrid” circuit which limits the current. Circuit breakers exist, but they are part of protection circuits against lightning storms or such, or against accidents which would get mains lines in contact with telephone lines, so they won’t get tripped by mere short circuit. Even when they are tripped, those circuit breakers are resettable fuse types, and they return into conducting state after the surge current is removed.

  3. I’m a switch tech at a telco office. I can share my experience from a US based RBOC.

    If i see a line in PLO (permanent lock out) I don’t care. If someone puts a dead short on the circuit it will signal the the OE (office equipment) to go OFHK (off hook). If the short is there for too long the switch will change the OE condition to PLO. The easiest way to clear a PLO condition is to remove the short, or the offending resistance across the pair. Put high impedance on across the pairs and my switches don’t care.

    There was a time long ago, when Ma Bell ruled the world. Ringers or phone sets could be counted on a telephone line by the amount of capacitance or resistance, I don’t remember which or it could be both, that was measured on the pair. There were people that checked it because residents had to pay to rent or purchase phones from Ma Bell and where charged for how many ringers were in the house.

    Divestiture cured all that for the most part.

    Now what i would like to see is an SS7 hack that would cure the world of BS telemarketing calls. Or a caller ID based system that uses a lookup table and a deny/allow list.

    1. There are systems that do what you are looking for. I seem to recall as Asterisk project. The problem is that the telemarketers are getting really ballsy and spoofing random CID info. The latest trick seems to be spoofing a CID that has your local AC and exchange so you think the call is from someone around you. Also a lot of the skum sucking maggots now have the balls to leave robo messages on your answering machine. My solution was to turn my answering machine into announce only, telling people to call back later. RIght (smile) and not answering for 10 rings so it wastes a bunch of their time. My next hack may be as Asterisk project to let me go through the CDR and quickly and 100% accurately fill out FCC complaints. If you don’t routinely fill out complaints against the telemarketers, they will never stop.

      The ultimate solution is going to be the FCC cracking down on telcos, mostly the VOIP termination services, and if they are making more than some level of calls per unit of time, make them justify them, and inspect them to make sure they are using the proper CID data and allowing people to unsubscribe. It is sad, but the VOIP termination services seem to like these people because they use a lot of billable minutes. The offending parties would be pretty easy to spot from teir vantage points. Just a matter of getting the FCC to get them to do it.

        1. Just make it illegal to spoof CID unless the number you are spoofing to is a number you own and use (e.g. those cases where someone calls out and wants their CID to show up to the other end as their main office number rather than a specific employee)

  4. I had a switch on the bell on Ma’s phones decades ago. No upset no answer nuttin. KISS.
    It’s series cap and the coil. Ringer equivalence.
    The nasty buggers have moved on to our cell phones. I looked one up the other day on the net, California to Indiana. The number was not in service, ghost call.

  5. I think it’s great for people to experiment, but I think this is also a case of a solution looking for a problem. We simply unplugged our phones or or better yet, turned the ringer off. By using one of the cheap recorders, we could listen to any messages the next morning. Except for the occasional wrong call or prank call, all that was left was emergency calls which was the reason for answering anyway. As many others, we’ve gotten away from these types of phones, but our last line of defense was to simply have a phone with an integrated light and volume ringer control for when it was ringing, a display of the number calling, and also an integrated recorder for messages. These phones had become dirt cheap by the time we ditched our “house phones”. I did play around with this phone and add music on hold circuit, but I do remember the conspiracy of the phone company selling all these additional options like call back, call block, call forward and whatever else people were desperate to pay for just to be able to sit down and eat without being disturbed. Different world now, isn’t it? I still do love to see little circuits for stealing power off the line, a few simple LEDs running dim in dark places are great, especially if you can tap into the cable outside and route the wires to, say, some steps on a deck or a fake security indicator.

  6. If I were going to do this, I would want my answering machine to pick up.
    So…phones don’t ring after hours, but answering machine sees incoming call and answers.

    I think you’d want to split the incoming line to two connectors. One would have the answering machine (it’s always connected to the incoming line), and the other connector would have the phone(s). That connector gets shut off after hours. A relay, controlled by your ‘after hours” timer, would do the job. Since the answering machine is always connected to the incoming line, it always sees at least one ringer, so the CO doesn’t think your line is open.

  7. Two or three small 0.5W 24V light bulbs in series with 1uf/100V and a switch works great.
    Or just use a 60V T5 telephone slide Indicator lamp.
    And to completely disable it, 1K8 2W resistor instead.

  8. To avoid being woken by annoying middle-of-the-night phone calls when I was in college, I opened the (Genuine Bell type 500) phone and removed the brass bells. When the phone rang, it made a soft, purring noise. Loud enough to wake me, soft enough for me to roll over and ignore it.

    [caution: This trick won’t work with cheap electronic ringers…]

  9. I can’t think that I’ve seen a phone made since 1970 that didn’t have a volume control or ringer disable… little switch on the side, or kind of wheel thing sticking out underneath that disengages bell somehow, anything with buttons/display RTFM.

  10. If you call me after 23:11, you’ll hear (in spanish): Do you know what time it is? Local time in Madrid: 23 hours and 43 minutes. If it is an emergency, press 1. If not, call after 8:38. Bye!

    1. Others have pointed out pretty much the same thing. I believe the idea of the exercise is that when the CO thinks your phone has gone off-hook in response to the incoming call, the voicemail feature won’t be activated – so the caller can’t even leave a message. There won’t be any messages to listen to and erase the next day. A caller never gets ANY information past THIS blockade during sleepy time…

  11. For landlines, why not just put a switch on the yellow line to disable the ringer? Then, after n silent rings, the answering machine takes the call. Use a big knife switch for that classic touch.

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