[nsp] Mismatch Codecs on Existing Calls

[Andrew Melton]

Is it possible for a call between a 7940G IP phone and a Cat6509 6608 6-T1 blade, to have different codecs in different directions?  

We have noticed that when users call a fax or modem, the far end analog devices begins to transmit negotiation tones.  Those tones seem to be recognized by the gateway as a fax call and the codec is changed on the incoming audio path to g.711.

However, since it is a person on the phone and not a corresponding machine, modem/fax negotiation tones are never returned, seemingly not forcing the transmit codec to g.711.

What we see is a bandwidth hungry call coming into the IP network, using a g.711 codec to transmit to the user on the IP phone.  The return audio is a normal g.729 call.  The call works, and the user can speak and be heard, but the codecs appeor to differ on the inbound and outbound legs.

I would not have thought this possible...