IP-PBX - side:
In order to register a remote IP phone with an IP-PBX , you must forward the following ports. Inbound traffic on these ports from the Internet must be forwarded to the internal private IP address of your IP-PBX:
* 5060 (UDP). SIP signaling and call setup/teardown.
* 10000-20000 (UDP). RTP voice traffic. A voice conversation picks a random pair of ports in that range, so the entire range must be forwarded.
Disable SIP handling features on your firewall or router. They will interfere with VoIP traffic. Different manufacturers use different terms:
* SIP ALG / Application Layer Gateway. (Seen on Linksys)
* SIP Fixup. (Cisco)
* SIP Markup.
* SIP Session Helper.
* SIP Translation.
Turn off any of the above.
Remote user's side:
In addition to above, each remote user also needs to open the following ports on their (remote-side) firewall:
* 5060 (UDP), responsible for call setup and teardown.
* 10000-20000 (UDP), which are RTP ports used for the transmission of audio.