GSM-to-Skype Bridge Lets You Lose Those Roaming Fees

Here’s the scenario: you’re going to be traveling somewhere and you’ll be charged roaming fees if you use your cellphone. But there is free WiFi available in this place. You can save yourself money by leaving your SIM card at home and using a GSM-to-Skype bridge to take calls on your phone via WiFi.

[Trax] is using a USB GSM modem to take cellphone calls on a PC. He leaves his sim card in this modem so that it can make and receive calls and text messages through your normal telephone number. For some reason, the USB connection only provides control of this modem and doesn’t pass bi-directional audio. To make this happen, he built an audio interface cable using two transformers and a few passive components to connect the modem to the computer’s audio card.

On the software side of things, an application written in Delphi 7 manages the modem, the audio stream, and the Skype application. When a call is incoming it sets up a Skype connection with your handset via the Internet, passing along the caller ID data in the process. If you choose to answer the Skype session the application will pick up the GSM call and you’ll be connected. It works the same way when placing an outgoing call.

This seems easier to manage than a rig that physically pushes a cellphone’s buttons via the Internet.

[Thanks Mure]

21 thoughts on “GSM-to-Skype Bridge Lets You Lose Those Roaming Fees

  1. The reason why it can’t send audio through USB is because it probably only has a USB to UART converter and a barebones radio inside. It appears to be the same DealExtreme adapter covered earlier in a post about cheap GSM radios for Arduinos.

  2. There is nothing odd not about passing the audio via the usb port. GSM/GPRS modems don’t do that. And remember that the modem in question was featured here on hack-a-day as a easy way to add functionality to projects…
    The modem is also reasonably cheap ready to use solution. You dont have to dick around with powersupplies (1.8A peac current for my 14.70 eur module), level conversion, antennas and their connectors (fu MMCX) all kinds of unfun stuff does not have to be done. Should have done it like that myself.

  3. Actually, there are quite a few USB GSM/GPRS modems that DO pass audio through the USB port and Huawei K3715 is one of them. It is one of Vodafone’s offers (in some countries), and although the provided software does not show any signs of call support, the original Dashboard does enable call functions in some of the usb sticks, allowing voice calls, non IP obviously.

    Anyway, some operators also made available a computer application that will send and receive both calls and sms through Internet. (Meaning if your phone is powered off, the calls will be redirected to the computer. Calls received (even in roaming) cost nothing, and charges apply oh calls made just like if they were made nationally.

    Props on the hack though, nice to see it making use of skype! :)

  4. is it possible with this set up to example:

    in foreign country buy foreign gsm install it have it hooked up to usa skype account and foreign internet connection.

    then in usa dial skype number from any phone then it will on foreign pc skype setup answer then dial out using foreign gsm? correct ?

  5. wonder if one can use the GSM radio with prepaid sim to set up a BBS couple with some small linux distro + wifi card + optional rj11 bridge. and use it as a remote shell for auditing wherever it was dropped. Or even better… bridge it inside someones telco and passively record phone conversations, while sniffing their and their neighbors network traffic. Or drop it on a PBX and got yourself inside access on the network. Guess what I really am suggesting is a cheap and fast network tap, that is quick to implement and hard to trace.

  6. Rather than physically looping the ausio out back into the mic socket could he not just set the recording properties to record the ‘stero mix’ channel?

    I do this on WIn XP, Vista and 7 (more difficult on 7) and i think this should give the desired result.


  7. Skype is being bought by MS, what will happen to it and its pricing is completely unsure, but that MS is then seeing all your communications contacts is though..

  8. Well, the number isn’t available outside the US for google voice, but if I’m using wifi, would that matter? If I go to a starbucks in Germany, turn on wifi on my phone and kick up google voice it’s just bits through the cloud. I think the phone number integration is what’s not supported outside the US for Google Voice.

  9. The idea might be nice…
    However, you can get an sip account with a landline number for free in many countries.
    Your smartphone should be capable to run a sip-client software.
    Wherever you are in the world, mummy can call you on the local town number (possibly for free) which you received from your sip client provider.
    And well don’t forget. Running a PC for this as a the only purpose isn’t very green and the electricity bill might eat up a good part of the money you saved on roaming fees

  10. It’s just an idea, I also written on, what about using another telephone like a modem? For ex. I have a Galaxy S – which I would use abroad with Skype and a Sony Ericsson K750i that can act as a modem (even on BT). Would that work? Or better, using MyPhoneExplorer (for the K750i)which takes control over the phone. What do you thing about this?

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