Situation: 3CX 15.5 hosted on AWS; 5 Yealink T42S phones successfully provisioned and deployed on customer's LAN. Inbound and Outbound phone calls all work very well. Our problem is that when internal extension call another extension there is no voice head. For example. Extension 111 calls extension 112 and when answered there is no sound. Both extensions show BLF, show the other extension as busy, etc. just no sound. As a test I took an extension to my office so it is on a 100% completely different Lan and Wan connection. No problems at all with extension to extension voice calls. Repeated same test with softphones with 100% success. Since signalling b/t phone works but there is not sound i am guessing that RTP ports are being blocked somewhere and i would appreciate any guidance on what to look at. Is it a port? is it a codec? Thank you in advance. please let me know if more information would be helpful.