Hi
Can you try your SIM card in another phone as a process of elimination.
I assume you have a 5s as a 5 won't work on o2's 4G spectrum
iPhone Reset.
On the iPhone hold down the Sleep/Wake Button and Home button together until the Apple Logo shows. Then On seeing this Apple Logo release both buttons ,and the iPhone will reset , which has been known to clear most glitches.
Other options are :
Settings -> General -> Reset -> Reset Network Settings.
Settings -> General-> Reset -> Reset All Settings.
Another option is to visit an O2 shop and they can perform a sim swap for you incase the SIM card is failing.
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.
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 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.