Bad MOH quality at beginning of call


I’m running Asterisk 1.2.17 and Linux with ztdummy and I’m seeing some strange MOH behaviour that I can’t explain.

I have two indentical Asterisk PBXs which are remote to each other. When one calls the other and is answered, then gets put on hold, the caller sometimes hears “robotic” sounding music on hold. Over the next 20 seconds or so, the call quality gradually improves to normal levels.

Interestingly, this never happens when calling from the same LAN.

The part that has got me baffled though, is that the actual handset that answers the call seems to affect the music on hold quality. When answered on a Polycom 601, I can get it fault every time. When answered on a Cisco 7940, I can very rarely get it to fault. This is on the same switchport with no duplex mismatches with either phone.

Why does audio quality start off bad and then recover after a while?

How can the handset influence music on hold quality?

All reinvites are disabled and Asterisk sits in the callpath. G.711 alaw is in use end to end.