Either block the number from your phones settings or give O2 a call
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.
*The Game Is On*
O2 recently upgraded their voicemail system and some people have noticed some issues since.
I'd suggest turning off your voicemail and then reboot your phone, turn voicemail back on and see if it still happens.
If it continues, call customer service to reset your voicemail account.
Check in the phone app settings under call settings >voicemail
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