What to do in case iphone doesn't play ringtones on sleep mode?

I had the same problem with my brand new phone. The problem was that it would go to sleep and the call wouldn't come through unless you called twice. The second call had to be before the phone went back to sleep. Then I simply took it to Apple store, they said it is related to hardware, they replaced it and the new one is fine. You can try this: Network: Normally it is on automatic as you are on roaming it will continue to switch between several available ones. Mail checks: turn it to manual as while checking for mail, can miss calls which suddenly reach by the fifth ring of the caller. If anything doesn't work try resetting the phone and test it to see if the problem still persists. Good luck.