Have you tried rebooting the phone @Reinhard (Turning it off and on)
If that doesn't work then you need to call O2 to send you a new sim
Numbers here Guide: Coronavirus Community Help and Support
Advice from an O2 Guru regarding a way to get through:
If you call 202, when it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to an advisor.
Press Upgrade, Lost/Stolen or Fraud if you have to.
Try turning off and remove the sim.
Wipe it carefully with a sodt dry clotch then reinsert it.
If it continues to play up, request a new sim as advised above.
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