I had an issue recently after O2 upgraded my tariff where the “Network Provider Voicemail” notification appeared on my phone and couldn’t be cleared. After 6 support calls with both Apple and O2 I eventually found a solution myself so posting it here in case others find it useful.
In summary, you need to dial 901 and switch your notification method to SMS then switch it back again.
Step by step:
Dial 901
Press *
Press 5
Press 1
Hang up
Dial 901 again
Press *
Press 5
Press 3
Hang up
This worked for me, your mileage may vary ✌️
