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Most cellphones today have an audio jack that works as both output and input for sound. That's why there's headphones today with built in microphones for making phone calls. Now then, is it possible, using this two way audio jack, to make a dial up connection from my PC (using a software modem) through my phone?

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Most phones actually have a serial port accessible over USB that has the same interface as a serial modem and long ago this was used with laptops to access the internet via dial up. –  Dan D. Apr 30 '14 at 20:06
Someone can call me out on this, but I'm fairly sure the data being send over dial up connection is more than just audio. –  Moses Apr 30 '14 at 20:11
@Moses POTs can only transmit audio. –  Elliott Frisch Apr 30 '14 at 20:17
It is a good technical question, I think, but why do you need it today? Is it only way your computer can be connected to Internet? –  Nikolay Apr 30 '14 at 20:22
Tethering would make more sense. –  Joseph Apr 30 '14 at 20:24

3 Answers 3

No. Analog telephone modems (a.k.a. POTS - "Plain Old Telephone Service" modems) worked because when the traditional telephone network (a.k.a the PSTN - "Public Switched Telephone Network") digitized the audio, it did so with 8-bit samples 8,000 times per second, for a total of 8 bits * 8 kHz = 64,000 bps digital audio. It wasn't possible to fully utilize that full 64kbps for data because of problems with the analog <-> digital conversion (ADC/DAC), but if you had a digital modem on a digital phone line (like an ISDN B-channel), you could send at 53-56kbps through a single DAC. But from the analog line, you could only send 33.8kbps through a single ADC.

But the problem with cell phones is that they've never provided a full 64kbps digital audio channel like the landline PSTN did. They use lossy codecs to compress your voice audio down to fit into a 9600 bps channel. So if you want to try to do audio-modulated signaling over that channel, I think you'd be lucky to get 4800 bps (4.8kbps = 0.6 KibiBytes/sec) out of it. And that's assuming you design custom modulation schemes to get the most out of what that audio channel gives you. I have no idea if any of the traditional modulation schemes used by POTS modems would work well over GSM voice channels. For all I know, an off the shelf POTS modem may just completely fall on its face and not even be able to sustain the ages-old 300 bps rate over a GSM voice channel.

Update: I found a reference online that said that doing analog audio modulation over GSM voice channels was typically limited to 2400 bps, so my estimate above was quite generous.

This goes a little outside the scope of your question, but note that technologies existed to make dialup-like connections over digital cellular networks but without the severely rate-limiting analog complication of hooking a POTS modem to the headset jack of the cell phone.

One solution was called "Circuit-Switched Data" or CSD, and usually involved hooking your computer to your cell phone via a serial connection of some type (using traditional RS-232 or RS-422 style serial interfaces or IrDA in the old days, and USB or Bluetooth more recently), and having your cell phone give that serial interface direct access to the 9600 bps data connection to the wireless carrier's equipment. Inside the wireless carrier's network, your 9600 bps data stream would be fed into a digital modem hooked up to a digital phone line, so you could use this technology to make a 9600 bps modem call to any modem on the PSTN.

CSD was followed by High-Speed CSD (HSCSD), which was like CSD except your cell phone would use 4 GSM voice channels simultaneously, so you'd be able to make ~56kbps connections, just like (in the downstream channel at least) the last era of POTS modems (V.90).

Over digital cellular technologies like GPRS and EDGE, it was possible to use that serial connection to the phone to do PPP, but that wouldn't have worked to just any modem hooked up to any phone line; the other end of your PPP connection would need to be a PPP server that receives its PPP connections not from modems, but over IP, tunneled in L2TP. This was common among ISPs that outsourced their dial-up modem POPs to third parties like Level 3 and UUNet, and is still common today among DSL ISPs (that's why so many DSL services require PPPoA or PPPoE).

tl;dr: To answer your specific question, No, it is probably not possible to hook a POTS modem via headset jack to a cell phone and make a successful connection to a modem on the PSTN. If it connects at all, it will probably only use extremely low data rates (even as POTS modems go), and be glitchy and unreliable. However, there are (or have been) other ways to connect a computer to a cell phone to make dial-up modem calls or PPP connections.

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This is a problem for analog faxes on digital phone lines as well –  Keltari Apr 30 '14 at 20:45
VoIP uses a similar behavior where if a few bits are off it just doesn't and raises it hands :-) –  Ramhound Apr 30 '14 at 21:14
I'm not sure I understand this answer. @Spiff starts saying "No", then goes on to explain how it actually can be possible, even though highly difficult to implement and getting a very low bandwidth. Right? ... –  msb Apr 30 '14 at 22:35
@msb I see my Answer as basically a No, but it's a qualified No. :-) –  Spiff Apr 30 '14 at 23:23
@Keltari: That's why GSM has a Fax protocol too, besides Voice and CSD ;). GSM was truly designed as a digital network, with the intent to do more than just voice. –  MSalters May 1 '14 at 11:09

Not a direct answer but a curious one.

While it is impossible to use legacy modems with cellular voice channels, it is, in fact, possible to design a specialized modem which will work over it (more or less by plugging into a handset jack, as OP desires). This is a useful feature, because:

  1. Data coverage may not be available in the remote areas where data communications may still be required (to support ATMs and EFT terminals, to name the most common need).
  2. Voice channel has highest communication priority and will keep operating even if data services are completely overflown (by a massive crowd or spike in usage).

IEEE article describing a possible approach:


A company which managed to turn the above approach into a successful commercial product:


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There are other useful situations for this too. What I happen to be looking for is not to connect to the Internet, but rather to connect to and administer a legacy phone system (PBX). –  druciferre Jul 30 '14 at 15:31

if you are talking about converting your phones Data connection to the cell provider into a dial-up signal to input into dial up modem, this may be possible and would probably overcome most of the issues that others are talking about, if you were trying to use the cell phone as a dial up modem that would be a different story.

if you could figure out how to convert your data connection to the same signal you would still have to find a audio jack to phone line adapter and then plug it into a dial-up modem, in which you would only be able to achieve the data speeds of the Dial-up modem

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Could I theoretically use a combination of these: showmecables.com/product/… ebay.com/itm/… in order to hook up my phone to the modem? –  Generalkidd May 1 '14 at 17:46
yes, I think so, you would have to have the right software on your phone to translate the Data connection to the correct signal, and to take the input from the modem as well. I think the software thing is going to be the tricky part of the whole set up really. –  Malachi May 1 '14 at 18:05
What if I used a dialup program from the ISP like NetZero? I know what number the program will dial, so I can just dial that number first, then run the program. It will dial again, but that shouldn't make a difference, the call should still connect right? –  Generalkidd May 1 '14 at 18:25
it's not about the call connecting, I think you are still thinking of dialing into your phone. what I am thinking about is bypassing that and turning the headphone jack into a phone line that translates your digital data connection from your phone into analog signals and the other way around to transmit. does that make sense? –  Malachi May 1 '14 at 18:34
Well in this case, I'm assuming there's no wifi available and no 3G/4G signals available either. All that's available is the ability to make phone calls. What I'd like to do in this case is use my phone to dial out to NetZero or some other dial up ISP and use the audio line to a USB modem to provide dial up internet service to my laptop or tablet. –  Generalkidd May 1 '14 at 19:25

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