I have just set a system up for a customer. Their server is held in a data centre and I am unable to use the 3CX tunnel as the customer wants a minimal amount of kit on site. I initially set the extensions to connect over the external address and all was well. That is until someone tried to make an internal call at which point there was no audio. I re-read the article and have tried making the RTP ports of the phone unique but this has not helped. For now they are going down a site to site VPN as local extensions and working fine. The tunnel however keeps dropping cause of a firewall issue so they need to be set up to avoid the tunnel. I am not using STUN as there is a static IP, however did initially test using STUN and had the same effect. The customer is using Snom 720s. Any help would be appreciated.