Because of the large number of supported codecs, since firmware version 8, some SIP requests leaving the phone (often a SIP INVITE) can exceed the maximum size of an IP packet that can be transmitted without fragmentation.
Some routers, DSL lines or VPN tunnels cannot handle such large packets which causes the call to fail.
Depending on the MTU size your network components can process, in some cases large packets are required to be fragmented.
This can be achieved by enabling an internal setting named ip_frag_enable.
This setting isn't managed via Web User Interface, so you can change it via provisioning or via setting change HTTP GET request:
Note: If you have set up user name and password to protect the webinterface, Hidden tags need to be disabled in order to have the setting change taking effect.
In order to send a HTTP GET request to the phone, you can use any browser on your PC. To do this, copy the above request and paste it in the address bar of your browser. Replace phoneIP with the IP address of your phone, then press Enter on your PC to send the request.
Starting from version 188.8.131.52 the default value of ip_frag_enable is on.
If you have tried the above solution and you still have the problem, here are some additional things you could try:
1. Remove some of the codecs in the codecs list under Identity -> RTP Settings(Section): Codec, this will reduce the size of the message to avoid fragmentation. For example if you know that you are using only G722 and PCMA you can set the codec list to "g722,pcma,telephone-event" (please leave "telephone-event" in the list)
2. Try to use TCP/TLS instead of UDP if your server supports it, as this will avoid fragmentation issues because the TCP protocol works differently than UDP. To do this, you need to set "SERVER_IP:PORT;transport=tcp" or "SERVER_IP:PORT;transport=tls" in the Identity -> Outbound Proxy parameter, for example:
- Outbound proxy: 192.168.1.2:5061;transport=tcp
3. You can also try to enable short headers on the phone by setting the following to on: http://wiki.snom.com/Settings/short_form. This will reduce the size of the SIP headers in order to avoid the packets from reaching the fragmentation size. It could be that compact headers are not supported in your SIP network, so if registration fails after setting this, please revert.
4. Check which hop in the network does not send the message further correctly. Check the fragmentation settings for this hop and see if you can adjust them to get the packets from the phone forwarded correctly.