Can not hear receiver during call

After a few minutes of a call, we can no longer hear the receiver. They can hear us; we can’t hear them.

Asterisk 1.4.5
Our phone: Grandstream GXP2000 Enterprise IP phone

Their phone is through Asterisk using one of the channels in our T1. We are using the Digium T1 card (TE120P).

Linux 2.6.latest
Processor AMD Opteron (load is low).

Any suggestions?

Our call plan works pretty good. We are now struggling with some issues like this on and we can’t seem to work them out.