This information very important to all the user of the mobile phone who are using VoIP.
If you notice that after a few minutes and you can no longer receive calls you likely have a timeout issue. This can be fixed by using advanced settings and setting up a STUN server.
1) Open Domain parameters, then select your SIP server domain, it should be there.
2) STUN Settings (for a list of public servers google 'public stun servers')
STUN server: ex. stunserver.org
Port: 3478 (or whatever your server uses)
Shared Secret: Off
Start Port: 49152
End Port: 65535
3) TCP NAT bind refresh timer:1200
4) UDP NAT bind refresh timer:28
5) CRLF refresh: ON