Thanks to ReallyWTF, as i had the same problem with the phone screen going blank on dialing a number and not being able to ring off. The only cure was removing the battery or holding down the start button until the phone restarted. The problem was caused by the screen protector not being correctly punched out at the proximity location. Removed the screen protector and pushed out the plastic that should have come away after punching and then reapplied the screen protector. The phone now functions correctly and the screen does not go blank while the number is being dialed.
Many thanks for the pointer to what might be the cause as I was getting prepared to contact the phone supplier and ask for assistance.