Disable SIP ALG in firewall
What is SIP ALG?
SIP ALG stands for Application Layer Gateway and is common in all many commercial routers. Its purpose is to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it.
Many routers have SIP ALG turned on by default.
There are various solutions for SIP clients behind NAT, some of them in the client side (STUN, TURN, ICE), others are in the server side (Proxy RTP as RtpProxy, MediaProxy).
Generally speaking, ALG works typically in the client side LAN router or gateway. In some scenarios, some client-side solutions are not valid, for example, STUN with symmetrical NAT router. If the SIP proxy doesn't provide a server-side NAT solution, then an ALG solution could have a place.
An ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol packet-inspection of traffic through it. A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signalling and audio traffic between the client behind NAT and the SIP endpoint possible.
How can it affect VoIP?
Even though SIP ALG is intended to assist users who have phones on private IP addresses (Class C 192.168.X.X), in many cases it is implemented poorly and actually causes more problems than it solves. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behaviour, such as phones not registering and incoming calls failing.
Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled.