I was experiencing this annoying ringtone issue since day 1. Nearly 9 out of 10 calls used to ring with the default nokia tone. Even the ringtone patch or changing the call log settings didnt help. Had the phone automatically swithching off bug too.
So got a firmware upgrade (V 5.0609.2.0.1) 01-03-06 RM-84 but the ringtone issue was still there. Applied the ringtone patch after the firmware upgrade and alls well now. No ringtone or switching off issue.
My phone is stuffed with applications music and games and I have no issues at all. The firmware upgrade was free and it is if your phone is under waranty period. Got my phone back in an hour.
The firmware upgrade changes many things as mentioned somewhere in this forum but there are many changes which one can immediately make out. So i would advice nokia N70 users to go for he upgrade.