![]() (see all the port specifications and firewall related recommendation) (Read the Doc and See page 4 of 5 and the topology 'BT Supplied PBX Connected to BT Net Services' This is the least intrusive method (since Firewall doesn't modify the SIP header) and it uses Proxy/IP-PBX Ideally with a Firewall your deployment should be based on the following document from BT. VoIP SIP Server (Proxy/Call Manager/Registration Server/IP-PBX) is in the LAN and it should be configured as 'behind a NAT Device (here Firewall) Recommendation 1: Your configuration of NAT Policy (for port forwarding SIP VoIP) is only suited for the following condition:. ![]() Since NAT ignores the SIP header, the Firewall is unaware of the endpoints embedded in the SDP header to apply any translation for RTP (Voice/Video) traffic part. Audio is due to the Firewall not being able to identify and forward Voice related RTP (UDP) Traffic that is received and to be send to the correct end point because this information is embedded within SIP protocol header using SDP Session Description Protocol and this contains Private Port and Private IP information which bypasses NAT since SIP is an Application Layer 7 Protocol and NAT is a function that works in the Network Layer 3 and Transport Layer 4. ![]()
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