If you have a static (or de facto static) IP address and your provider supports it, use IP authentication instead of registration.
Strange; I believe the default is 10000.
The usual cause of this issue is a ‘poisoned’ NAT association kept alive by aggressive retries. Router/firewall make/model? In general, you want to disable any SIP ALG or source port rewriting and enable consistent NAT. The UDP ‘unreplied’ timeout should be less than half the retry interval but the ‘assured’ timeout should be at least twice as great. If you can’t control these, try setting all three Retry Intervals and Qualify Frequency to 600. That should allow sufficient time for the router to time out any bad entries.
If you still have trouble, look at whether the log shows no reply, or an error response, to the failed register attempts.