This issue has a particularly bad effect when installing VoIP services behind a Sonicwall firewall

In our scenario, we were installing a 3CX Phone System

We could make successful outbound calls but inbound calls were sporadically successful.  Sometimes inbound calls would work fine, then other times they would not

We were aware of an issue with Sonicwalls where you are required to increase the Default UDP Connection Timeout value from 30s to 180s

‘Firewall Settings – Advanced – Default UDP Connection Timeout (seconds):’

Increasing this Default UDP Connection Timeout value to 180s resolves the following issue:

Many VoIP Phone Systems will perform a SIP registration every 120 seconds with their ITSP (In some case longer but typically not less than 60 seconds).  This is an outbound initiated connection utilising the UDP protocol.  By default Sonicwalls will terminate idle UDP sessions after 30 seconds (As stated above).  The effect of this is that following SIP registration, inbound calls are successful for the first 30 seconds.  After 30 seconds, inbounds calls will fail (Assuming UDP connection has been idle) until the system reaches its SIP registration interval which is typically not less than 60 seconds, often 120 seconds, resulting in a period of failed inbound calls.  My mobile phone offered a ‘User busy’ message in this situation

Now on this occasion, changing the Default UDP Connection Timeout value did not fix the problem

After some frustration  we discovered that any existing firewall rules inherited the Global UDP Connection Timeout at the point of creation.  Therefore, all existing firewall rules had a UDP timeout value of 30 seconds.  In particular the default LAN > WAN rule (ie any source - to any destination - on any port - outbound – allow) found here:

Firewall – Access Rules – LAN>WAN

If you edit this rule, access the Advanced Tab and change the UDP Timeout value to 180, the problem will be solved!

Our chosen ITSP is who, as always were extremely helpful in assisting us in resolving this problem.  Highly recommended!