Need sugestions / instructions for analog modem

I am looking to be able to use a regular old fashion modem on a SIP connection. I would like to assign a DID to a device (ATA or other) and be able to get the modem to dial and work with a PC using the standard com ports.

Does anyone have any suggestions on what hardware I should be using for this? Will we have issues similar to some of the FAX reliability issues?

In this particular application I will have multiple PC’s that will need to be able to utilize this solution, although we can live with one at a time wjich is what they have currently by all sharing a single analog POTS line. The PC’s currently are using a single analog line and a regular modem. When I convert them over to FreePBX and port the analog lines over to SIP, I would like for them to still be able to use the modems.



I have seen people suggest using G.711 but that would probably turn out to be as unreliable as it is for faxes.

I am not sure if I can post links to some of the things I found since Sangoma might offer something to address this but I suggest you google “Modem over IP”. Looks like there are companies which specialize in that…

Good luck and have a nice day!


Modem’s will only work over g711 they are harder to get working than faxes because there is no T38 for modems, so it totally depends on your VSP, chances are if you are only needing an ATM machine/postal meter/credit card reader then 2400 baud will work with most . the higher the baud rate, the higher likelihood of errors. If it doesn’t work with your VSP, then the only option is to try another.

A PC’s standard COM port (the standard DB9 connector) can only be connected to a physical modem, which you can then connect to an ATA’s FXS port via the standard RJ11 connector, if you do so you will need to disable any echo cancellation on the ATA.


Apparently T.38 “equivalent” for modems is ITU V.150.1… It’s apparently not that well supported…

Have a nice day!