Intermittent audio issue

Hi FreePBXers,

I’m running
Asterisk 1.6.2
On Centos 5.5

My system has two interfaces, one with an Internet facing static IP (Firewalled to exclude connections from all but my VoIP provider) and the other is an internal interface that has my endpoints connected.

There is no NAT involved and I’ve set Asterisk and all endpoints up with NAT=never and no reinvite.

I’m using Yealink T22P/T28P phones and they sit on their very own network (separated from the main network by a Router).

I’ve had my ISP set COS on the fibre connection to prefer VoIP traffic and call quality is generally very good.

The issue I’m having is that during longer calls the audio does a 2-5 second drop-out but only on our side.
This seems to occur when the external caller talks for long periods of time but I can’t say for certain that that is the cause. It does appear that if I say something or make a noise it brings the audio back quicker.

This leads me to think that this is an RTP issue and I have the server set with.

300(rtptimeout) 320(rtpholdtimeout) 15(rtpkeepalive)

I’m really not sure where to start to diagnose this issue so would appreciate some pointers on where to begin. I’m leading towards thinking that it may be a setting on the phones themselves but I’m a bit hesitant to play “Trial and Error” on a live system.