Reading other theads from other sites the more common fixes are:-
- Turning Airplane mode on / off
- Making sure Service Provider (Under Settings) is on Automatic
- Doing a hard reset on phone (hold Home and Power till Apple logo appears)
- Turn phone off, remove sim for 5 mins and reinsert and turn phone back on.
That last one seems to be the most common fix around other sites
Or or it could be a locked phone meaning it has been restricted by software so that it can only be used with a single network or carrier. Locked mobile phones are usually GSM phones that use a SIM card