3GS wont ring on stand-by mode.

i have this iPhone 3GS 5.1.1 Factory Unlocked, everything works great, Good Service, Good WI-FI, Good Charge, but the problem is that only rings on incoming calls or text if lcd screen is on.

it will not awake or ring if on stand-by mode, i had to press the home button to awake then rings or notify that i have lost calls or messages.

i tried whit another SIM card and still same problem, only receive calls when on lock screen or home screen, or using it.

Restored on iTunes and installed this system board on another good working iphone and still same problem, so it has to be whit the board.

still same problem,anyone ?