Remote Jabber users connect to corporate network via AnyConnect VPN client. We tried PC's iPads and iPhones devices. The can IM with each other, they can see their presence status as well. When they try a voice call, the calls are processed and rings the called party but there is no audio.The Jabber client can call any other devices on corporate network without issue.
We had one user VPN through another corporate firewall and the call goes through and they have full 2-way audio.
Did some testing and found the 2 devices could not ping each other. Checked Firewall config and added same-security-traffic permit intra-interface. Users were able to ping each others VPN address, but still no voice audio streams during a voice call.
It sounds like your RTP ports are being blocked by the firewall. Once the call is connected the RTP audio stream uses UDP ports in the range 16384-32767. The first thing I would check is that this port range is open in the firewall.