As customers we can only give advice
You could try a replacement SIM as they are known to go faulty. Best calling in to an O2 shop with photo ID for that.
However, if the phone is fine in other areas then it looks like a network issue.
Download and register for My Network App where you can report faults and receive updates.
Guide: How can I sort out my network issues?
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
