I have an N97 which was originally Vodafone branded. It is now debranded and used on the o2 network with the Simplicity 20 SIM.
I don't know why, but the phone rings for about 5 secs and then goes to the answerphone. Really annoying as that's just not long enough for me to get it unlocked and the answer it, especially if it's in my pocket.
Just rang o2 and they gave me a code to enter that extends it out to 30 secs before answerphone kicks in. I don't know if it's N97 specific or o2 specific, but thought I'd share the code.
Just type in **61*901*11*30#
Hope this helps someone.