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Modem over IP/ Broadcasting - Comrex

We have a campus radio station which uses comrex blue box devices to broadcast audio.  These comrex devices work fine over direct telco analog lines but have various problems on our VOIP system.  I have it connected to a VG 202, have modem passthrough enabled everywhere.  Calls connect and seem to stay connected but constantly 'retrain' during which there are silences in the broadcast.    I contacted the comrex vendor and they were not very helpful, just indicated it will not work over VOIP.  Why?  Is there any way to make this work?   It uses a synchronous POTS codec.  Is there any tweaking/tuning that can be done either on the VG 202 or gateway routers (all MGCP) to improve performance?

Call traces do not indicate problem, CDR does not show jitter/lost packets/latency, etc...

According to comrex documentation retraining occurs 'if codec loses sync' or 'if it senses errors on line'.

Any help would be appreciated. I  have been pulling my hair out on this!

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