This is a very peculiar case which only occurs under specific circumstances.
We arfe using a Cisco 10.5 Callmanager together with 3rd party queue software from Imagicle.
Calls that are forwared to the 3rd party queue are completely silent when they eventually reach a phone.
Calls that go directly to a phone are pass a CUCM queue first don't have this problem.
I strongly suspect it has something to do with codec negotiation as the 3rd party queue software only uses G711. Codec settings on the TG800 has been set to reflect this but without success.