Business support

Using modem calls with VoIP

A modem translates information (binary data) from a phone or a cable line (analog channels), changes the data to a digital form, and delivers it to your computer.

While modems were created to be used on the traditional (analog) telephony network, you can configure them to work with VoIP (digital). Here are some troubleshooting tips to help ensure that your modem works correctly with VoIP.

To use modem calls over VoIP:

  • Configure the modem speed so that it doesn’t exceed 33.6Kbps. Modems using VoIP can run at rates above 14.4Kbps, but when using VoIP, set speeds lower.

  • Turn off V.90 and V.92.

  • Use the modem pass-through technique for handling modem calls with VoIP (CenturyLinks supported). (This is the same technique that CenturyLink supports for faxing with VoIP.) A modem call with VoIP using the modem pass-through technique involves a sending modem and receiving modem to communicate directly.  The sending modem calls the receiving modem, and the call is converted to VoIP during the transmission.

The other technique for handling modem calls with VoIP is called modem relay. In the modem relay technique (not supported by CenturyLink), a modem call over VoIP uses the VoIP device, generally called an analog termination adapter (ATA), to spoof a modem on each end of the relay. This technique is analogous to the fax relay technique for handling fax calls using VoIP.