Cheers Steve, that's very helpful.
It looks like the DVMEGA AMBE3000 can generate AMBE comfort noise packets if it's not receiving any audio, I wonder if that's what's going on here-
The Voice Activity Detection (VAD) algorithm along with the Comfort Noise Insertion (CNI) feature of the AMBE-3000™ Vocoder Chip performs useful functions in systems trying to convert periods of silence, that exist in normal conversation, to savings in system bandwidth or power. VAD and CNI can be enabled by either hardware configuration pin (DTX_ENABLE TQFP pin5 BGA pin C1) or as part of a control packet.
I will drop Guus a message and see if he has any thoughts.