It could be there's a problem with the signal delivery at your location. It's possible your phone may be faulty and it's rare, but there could be a fault with your SIM card. Here's what to check.....
Do you have coverage at times, not at all or is the signal just weak?
Does anyone else in your vicinity on the O2 network have the same problem?
If you're indoors, check what signal strength is like outdoors or what does your screen show after you've moved location?
Have you tried switching your phone off and on? Better still, do that and remove and reinsert your SIM in the process.
Have you tried your SIM card in another unlocked phone or the phone of someone else on the plan.com network? Doing so will help identify where the problem sits, if there is a problem.
If you are overseas, have you tried manually selecting another available network to connect to, through your settings?
In the UK and poor signal strength indoors is the cause, if you have broadband at the premises in question, WiFi calling could be an option for you or you might benefit from a Boostbox.
Contact your account managers for advice or our customer service team on 03300 881818.