VP530 sending H264 traffic on voice only call

[aboodoo]

When a T21 calls a VP530, the calls sometimes drop. It never happens if a VP530 calls a VP530 or even if a T21 calls another T21 across locations. The drops happen roughly 50% of the time.

The failed calls are taking place across a LAN-to-LAN VPN connection and the T21 and VP530 are registered to different PBXs. If a T21 calls a VP530 on the same PBX, the calls do not drop.

I see in the Wireshark logs that the VP530 is sending H264 traffic and G711a on the voice only call and when the remote PBX receives the H264, the PBX disconnects the call.

I would appreciate any assistance in resolving dropped calls.