Hi
Do you get a No Network pop up ?
Look at your mast status
http://status.o2.co.uk and tel O2 Customer Services. 202 for Contract. 4445 for PAYG. 8002 for Business to have your account looked at.
Try Turnjng the phone off , clean the sim and reinsert and turn back on.
Lastly a sim swap is known to help.
SwapMySim Procedure.
If a person upgrades to another phone and the new phone takes another sized Sim Card then this procedure moves the Tariff and Mobile number from the original SIM card to the new one. If on PayAsYouGo any credit will move across too.
This is the same is a new SIM card is required if a person feels their original one may be at fault.
Any O2 HighStreet Store can perform the swap in store (if on Contract please take some Photo I.D just incase), or a person can take the SIM card home or request one via O2 Customer Services. 202 for Contract. 4445 for PAYG. 8002 for Business ->
http://www.o2.co.uk/contactusSim Swap link ->
http://swapmysim.o2.co.ukBe sure to ask for an Account Specific Blank SIM card as Contract and Pay As You Go versions hold different information.
The Service will stop on the original SIM card which indicates the swap has started. Turning the phone Off and On , or if applicable switching airplane mode On and Off ,with the new SIM card in every 20 minutes or so will typically help the connection to settle