The phone has O2 service but is unable to make or receive calls
Is there a crescent moon icon at the top of the screen, with the other icons?
If so, go to - Settings > Do Not Disturb > Off
If this doesn't work or the phone is not an iPhone -
Remove all diverts -
- Dial ##002# to cancel all diverts, including Voicemail 901.
- Dial 1750 to switch voicemail back on. Check it by making a test call to the phone.
If this doesn't work, go through the checks on this page.