I use Cisco SPA 122 to convert analog signals (eg fax signals) into a digital signal capable of being transmitted in SIP communications.

The problem is that when I plug in a postage meter or a credit card terminal, the center is never reached.

Do you know a way to make this transmission to work?

In France, these machines call surtaxed numbers like 0800 ... ...

1 Answer 1


You are basically using a MODEM in the device working over SIP. Modems work with modulated audio signals to indicate bits of data.

SIP traffic is unreliable (UDP) network traffic, data packets are not re-transmitted if dropped or received out of order (jitter).

Some of the codecs (Coder Decoders) can prevent successful communications because of the lossy compression, meaning the sound may have changed.

I'm guessing you may have heard phone calls that sound like they are "tinny". That is because of the fidelity lost in the compression process. It's like comparing JPG to GIF - JPG removes some of the data, but generally not enough to make it grainy. Or MP3 vs. WAV audio files - you wouldn't want to listen to 56k music, but for voice it would be fine.

A couple of things you can do to ensure success modem communications most or perhaps all of the time over SIP:

  • Use the G.711 codec - This gives you enough audio bandwidth to pass all of the modem communications tones. G.722 will be even better if available.
  • Drop the modem speed to 36K or lower. Lower speeds will be more forgiving to some packet drop or jitter.
  • Turn off error correction on the terminal device if possible. We have fax machines that run more reliably set this way.

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