Audio drop outs on VOIP calls, but only in one direction
We have a local PBX server powered by Asterisk software and we periodically (3-4 times per day) experience audio drop outs, but only on the external caller's side of the conversation. The caller can always here us but we cannot hear them.
When the drop occurs it affects all the calls that are active. We have 10 lines connected via a T1 MIS (managed internet service) connection from AT&T. If we have 5 concurrent calls the drop is experienced on all 5 calls. The drop out occurs for 10-20 seconds and then the audio comes back.
I have replaced the router, core switches, tested cables, and changed VOIP providers. Nothing has resolved the problem. PLEASE HELP. This has been going on for 5 months!