The phone has O2 service but is unable to make or receive calls

iPhone -

Is there a crescent moon icon at the top of the screen, with the other icons?

Do-Not-Disturb-Quarter-Moon.jpg

If so, go to - Settings > Do Not Disturb > Off

 

If this doesn't work or the phone is not an iPhone -

Remove all diverts -

  1. Dial ##002# to cancel all diverts, including Voicemail 901.
  2. 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.