I've got the following configuration : 2 subnets (we'll call A and B) with IP phones in both. Internet access behind a NAT for each of them, and a routed VPN is set up between those 2 subnets. I am using a hosted SIP provider (same for both networks). IP phones are configured to work behind a NAT and use the hosted SIP proxy.
Calls to the outside and incoming calls work fine, going though the hosted provider.
Calls between phones in subnet A are established directly, ie the SIP traffic still goes to the hosted provider but the RTP traffic stays local. Same for calls within subnet B.
Calls from A to B or from B to A work too. However the RTP traffic does not go though the VPN and makes a round trip to the SIP provider which is not ideal.
What options do I have to improve on this last point ?