I have a customer who uses the IP Communicator behind Cisco VPN into an older PIX. They can call normally whether extension to extension from VPN side to in house side, no problems, outside calls no problem, but when they try to call another VPN user with the same setup they get one way voice.
Previously they had one way voice to another facility that I was able to resolve. They have their private networks setup as class B in the 172. range, but not 172.16 like you would think, but actual public ip's. When they tried to call another building that had a connection inside, they had one way voice due to NAT issues, I resolved it by using a dedicated NAT router. What was happening was if a call was placed to a hardphone with an ip of say 10.10.20.1 from a softphone of 172.20.2.22, the call would connect but there was no NAT statement since the router doing nat was in front of the other location. The call would connect but once the callmanager dropped out the hard phone had no route back to the other phone and didn't know that its IP had changed. I think I have the same issue here, but have no idea how to resolve it.