Hello, I'm currently experiencing an issue with the 3CX application with internal calls (extension to extension). When an internal call is started, the application says "dialing" but does not play any ringing sound. The phone for the destination extension does not ring. Eventually the caller gets connected to the voicemail box of the destination extension. Looking at the 3CX activity, the PBX states that it is connecting the destination ext. but it times out. Here's a log entry for a call. For the bolded entries: [Extn] failed to reach Extn:1255, reason No Answer -- Again, it says "no answer" but the phone never rings. (IF THE CALL originates from a desk phone, not the 3CX application, the call is successful.) NAT/ALG check:L:3265.1[Extn] REQUEST 'INVITE' - some of SIP/SDP headers may contain inconsistent information or modified by intermediate hop SIP contact header is not equal to the SIP packet source(IPort): Contact address:192.168.2.26:61579 Received from :192.168.2.26:9590[/quote] --- I'm not sure why I'm getting this error. All of the internal traffic goes through a layer 3 switch (NO FIREWALL). Now, external calls from the 3CX application are successful. External/Internal calls from desk phones are successful, so the issue is very specific to internal calls that originate from the 3CX application. I've ran my test on Windows 8.1, but have confirmed the same happens on Windows 7. I do not understand why the PBX fails to ring the destination phone when the call is initiated by a caller using the 3CX app, but always successful for all other internal calls started by hard phones. Need some assistance understand what to try next. Thanks!