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Use Your ISDN Codec as a POTS Phone Patch by Dave Immer


New ISDN codecs and some legacy models can make and receive calls to and from standard POTS (Plain Old Telephone Service) telephones, and any phone available via the PSTN (Public Switched Telephone Network). Your ISDN line must be configured for Alternate Circuit Switched Data and Voice, which is what you typically get anyway.

It sounds as good if not better than standard POTS hybrid units such as Telos One or Gentner. And you will have the advantage of better metering and, in some units, digital i/o.

I have tested a handful of ISDN codecs that deliver a relatively clean POTS signal:
1. APT Tokyo & Equinox
2. Musicam Suprima & RoadWarrior
3. Telos Zephyr Xstream  & Classic

Other brands like TieLine and Mayah have the capability also.

Older models such as the CDQ Prima, PrimaLT, RoadRunner, APT Milano, 3D2 and DolbyFax do not have this capability, even if you do have a CSD/CSV enabled ISDN circuit.

But if your codec is able to initiate a call to a POTS phone, place the encoder in the G.711 mode and choose mu-law (µ-law) as the protocol (North America.) Some of the newer models auto-sense incoming Voice mode calls and adjust accordingly.

Let me know if you have questions or comments about a phone patch. Thanks,

Dave Immer,                                                             


Note from George:

I am always amazed how much better an ISDN codec works as a phone hybrid than even expensive digital hybrids like the Telos One.  I've mentioned this to Telos and they deny this function even exists!  Recently a client with a Telos Express (now discontinued) wanted to get a phone patch.  Since the Express has a built in mic preamp and mixer, it was a very simple process.  We plugged the monitor output of the Express into his USB audio interface, and a complete system was built with one cable.