When you have a Voip you can call and the onnection is build up. Now if you are being called the connection has to be active and your Voip server renews the connection each minute.
It is not stale it us waiting for a call.
Now you set the timeout to 30 secs what means that you are unreachable for also 30secs while the connection is not listening.
I have the same and my Voip reconnects every 5 mins and the default timeout is 3 mins. To avoid being not reachable for 2 mins. I put a backup connection NAT receiving calls on UDP 5060 and relaying it to the Voip server.
My advise it to return your default settingsand you need atleast 1 mins timeout to cover 1 mins renewal.