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Using an analog modem over a voip connection

madspire asked
For work purposes, I need to use an analog modem to dial in and connect to my companies phone system. Unfortunately, I do not have standard analog phone service in my home. I have cable internet, and use Vonage for phone service. This modem connection will need to occassionally be connected for several hours. What I need to know is, is it possible to connect through the digital line with an analog modem? How stable is the connection? Is there any chance of damage to the equipment? I did try to contatc vonage, but only got a canned response stating that this is not supported.

Appreciate any help, and espically advice from people who have/do do this -

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I recently ran into that same problem. We have a client who uses software which uses an analog model to dial into a server to transmit information. The phone system setup in the office was a VoIP system. I could hear the modem dial out but I would get all sorts of crazy noises on the line (almost like if you have ever hear a dial up with an accelerator). I would always get a generic modem error, and I could never get connected to the remote system. They had the phone company come in to install a regular analog phone line. I connected to it and transmitted the information just fine. I'm not sure if this would be the case in every single situation, but I'm pretty sure that using an analog modem with VoIP will ultimately run into problems.
Although Voip sound "just like" an analog line, the signal is digitized, compressed, and sent as digital data.
The analog modem uses frequencies that will certainly be "messed" up when compressed with such "lossy" codec schemes.

Maybe you could get it to work at really low baud rates (as in 1200baud) but even then, I'm not sure it would work.

Better find an up-to-date alternative to connect to your company phone system, as pure analog lines will become harder and harder to find.
A properly certified analog modem *should* have the same volt and current characteristics on its signal line as a standard telephone so connecting an analogue modem into your Vonage modem instead of a telephone should not destroy the hardware.

The Vonage VOIP service has packetization at 20ms intervals as standard. You can phone up Vonage and get them to change this to 10ms instead which shouldn't affect the phone service but should allow the analog modem to work [much better]. Phone tech support and ask them to change your packetization from 20ms ("high") to 10ms ("normal").

You could also try adding *99 to the beginning of the number you are dialling. This tells Vonage to do the following for THE NEXT CALL ONLY:
1) Change packetization from 20ms to 10ms
2) Disable "Bandwidth Saver" (gives the modem more bandwidth)
3) Uses codec G.711 (good for data).

I hope this helps.


I appreciate the responses, good and bad - points to all, with doneganm getting the accepted answer for the advice on how to get this to possibly work.

Thanks again -