When I enter the PIN, either of these would happen after the "code accepted" stage:
1. It continues working fine.
2. It continues but still cannot get signal.
3. It seems to continue with or without signal, but then immediately asks for the PIN again. Repeats 2-3 times until I get #1 or #2.
In #2, only a restart can make it work again.  Sometimes 2 restarts.

The problem could be either with the phone or the SIM.  I asked Celcom Careline in case they know about this.  But no, they just told me to test with another phone.  Before I get a chance to do that, I'd like to check first whether anyone else here has seen this problem.  Google couldn't find anything similar.

It happens intermittently at random times and regardless of my location.  Sometimes even during a call.  So even if I test it OK with another phone for one whole day, it wouldn't prove anything.
I can't simply take it to the NCC because I cannot reproduce the problem easily.
Been using the phone happily since August 2009 - this only started about a month ago.