Calls are diverted / number unobtainable - Callers cannot get through

[ak1-p]

I have an iPhone 12 mini (on pay-as-you-go). Some people who try to call me cannot get through and they are diverted to my voicemail. I turned the voicemail off in case that fixes the problem, but instead now those callers get a ‘number unobtainable’ message.

Most of those numbers are withheld numbers, although not all; I tried to call my mobile from my home phone (which is withheld) and I get the ‘number unobtainable’ message. Some calls get through, some others do not.

Also, I tried to switch my voicemail back on by calling 1750 and it does not work, but I would like to solve the above problem first.

Any advice would be gratefully received!

[MI5]

@ak1-p 

Settings >Phone >Silence unknown callers and turn Off.

Voicemail help here Guide: All you need to know about O2 Voicemail 901 

[ak1-p]

I am afraid it is already off. I went through all the settings. I think this problem may be coming from O2 rather than the phone itself.

[MI5]

@ak1-p 

You'll need to call customer service to check your account.
Numbers here Guide: Coronavirus Community Help and Support 
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 someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.