Not sure what the deal is... my 3GS which is NOT going into sleep mode after a missed call... It just sits there at the missed call number screen with the slide to unlock bar - and does NOT go back to sleep.
Before the obvious replies, yes the auto-lock is set to 1 minute. Yes, it DOES auto lock when left at the springboard icons unattended after 1 minute goes by. But it will NOT go back to sleep after I get a missed call.