I have modified my asterisk to measure how long it takes to register with the SIP server (usually central01). Over the past two weeks, I have seen spikes of up to 15 seconds that happen at least once a day. Even today after the database server maintenance, I had a spike of 14 seconds to west01. I have had better luck with west01 excepting today, with a spike of 6 seconds.

I experienced a 'not in service' message once. As has been previously mentioned, a delayed database lookup can cause this and also latency in registrations.