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.
Starting from version 18.104.22.168 the default value of ip_frag_enable is on.