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Modem doesn´t work with G729




I have a Callmanager v. 9 connected to a SIP-Trunk (G729). Also have some ATA conected to a modem in the envroment, the problem is that when call to modem´s DID the modem doesn´t negociated and never answer the call.

Do anyone have idea?





VIP Super Bronze

Have you tried with G711?

Have you tried with G711? Does that work? What is the region between the ATA and the sip trunk set to? is it G729 or G711?

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Just adding some info to this

Hope all is well Ayodeji. Just adding some info to this thread.

Using T.38 fax relay for fax-over-IP transmission is technically a more robust method of faxing and works better than other methods where available. But it is not available on all networks and to all endpoints, so fax passthrough (or fax through G.711) is still widely used because it interoperates easier and with more endpoints. If your provider offers T.38, investigate if failover to G.711 fax is offered as a secondary call negotiation service to connect calls to destinations that may not yet support T.38. Consider keeping fax on TDM trunks for a while longer if faxing is a critical part of your business.


Strongly consider getting G.711 service on your SIP trunk. Although it uses more bandwidth than G.729, it does not compromise voice quality (after all, SIP trunk technology is supposed to improve PSTN services, not worsen them). It also eases fax concerns, obviates the need for transcoding at your SBC, and better positions you for new SIP trunk services that will almost certainly include increasingly bandwidth-intensive applications such as high-fidelity wideband codecs and video. Dimensioning your network for G.711 call bandwidth now positions you better for future G.722 service, which offers much better voice quality at the same bandwidth use and is likely the first wideband codec to be offered on SIP trunks in the future.





HTH Regards, Yosh
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