Voice Calls Go Silence - Cisco 3750 Switches
I implemented QoS for a clien about 8mnths ago on when they implemented VoIP. They have 2 CORE SWs  and stacks of 3750s in the access layer.
They had implemented VoIP with no QoS enabled, then i had to help them when they were having some poor call quality. About a month ago, they have another problem, but not similar to the initial one. This time when i got to the site, the network guys were pointing at the Media servers, while the server guys were pointing at the network.
As i initial did QoS, had to check if all is in order and also check at the network health. The jitters, rtt, latency were very low, call MOS scores were perfect. On the WAN, packets were being matched and compression was perfect. All thelinks utilisation were never above 10% in the LAN and on their E1 links its averaging 55 - 60%. I had to capture packets and analyse but nothing seemed wrong except that i see some RTP packets in duplicate, difference only in the seq nnumbers.
Symptoms: After a couple of minutes on a call, u get a click sound, then the two cannot hear each other. The recordings on the Server proves that they cannot hear each other but you hear both parties talking trying to communicate. I am not a VOICE expert, but only do QoS on Switches and their Router and a few changes on the network.
COULD THIS BE A NETWORK ISSUE????? I NEED YOUR OPINION AND SUGGESTIONS ON HOW TO PROVE OR EXPLAIN THIS TO THEM.
THEY ARE USING CIC VoIP SYSTEM, 2 CIC SIP SERVERS, ONE IN BACKUP MOMDE, AND 2 MEDIA GATEWAYS LOAD BALANCING THE LOAD, AND HAVE AN AVERAGE OF 250CALLS DURING BUSINESS HOURS.