What phone do you have?
Have you taken the sim out and cleaned it and rebooted the phone?
@Tommy It's nearly 11 pm on a Sunday night. O2 do not operate a 24/7 customer service. Best time to call is between 8-8.30 am to avoid a long wait on hold.
Try switching your phone to 3g to see if that works?
If not, text the word "active" to 2020 then save the settings that get text back to you, reboot phone and try again.
You can also check for any local network issues here https://status.o2.co.uk/
Hi @Tommy, there's some good advice above, it'd be great if you could give it a go and let us know if it helps 
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