Issue with Extension to Extension Calling

We are running AltiWare 6.5.1.522 with IP710 and IP600 phones. The IP710 phones are running firmware 2194 and the IP600 phones are running 2091 firmware. The phones are not located in one physical office but rather spread out geographically on a WAN with a VPN. We have an issue where certain extension combinations are not able to talk with each other. The extension to extension call can be initiated, the phone rings, but once the receiver answers the phone, the caller and receiver can not hear each other. AltiWare shows the phones are still connected even though there is no sound. This is very consistent between certain extension pairs. However, these same extensions are able to handle extension to extension calls to other extensions on the system successfully. Between these trouble extension pairs, phone messages may be left on Altigen and retrieved if the receiver does not answer the initial ring and the call rolls to voicemail. Therefore, we know that the phones are properly connecting to the server. This problem exists with a few of the extension to extension pairs and it's always consistent in that it only affects certain pairs and not the rest of the calls from a trouble extension. One of the trouble extensions was rebuilt but the problem remains. We "moved" one of the trouble extensions to another physical location (re-registered the IP phone to a different IP address in a different office) and the calls were then successful. Phones and server have all been rebooted. Has anyone seen a problem like this before? Can anyone offer any suggestions? Thanks in advance.