'Phone restricted' help needed...

[UKDJ]

I just tried to pass down an old Nokia C2-01 to my elderly mother as the charging socket - not the battery! - on her existing hand-me-down Nokia 6300 has failed.
All appeared OK, I switched her SIM to the C2-01 & it accepted her unlock PIN - but then it says 'Phone restricted' & has 'Undo' at bottom of the screen?
I selected 'Undo' & then it says 'Enter restriction code:'... I tried her PIN again, plus the 'usual' '0000' & '1234', all to no avail. She only uses a mobile for occasional calls, but texts quite a lot (PAYG). Any ideas what else I might try???

[MI5]

@UKDJ 

It needs unlocking from whichever network has it locked.

If it's O2, how to here Guide: Unlocking an O2 phone to use a different SIM card 

but if she is using O2, it will be locked to a different network.

[UKDJ]

Thanks @MI5

My suspicion is that because the replacement handset has remained dormant in a drawer for many years it has been de-registered from the network. The odd thing is, when I opened it there was an old O2 SIM still in it! That SIM gave the same 'Phone restricted' message as when I put my Mum's current PAYG SIM in it.

I followed the link you provided, but failed the security checks (O2 didn't tell me which part I failed, but as my Mum doesn't really MAKE calls - only receives occasionally, and sends texts - I couldn't really 'provide 2 numbers you call regularly' which is one of the questions).

[MI5]

@UKDJ 

You need to contact O2.
All numbers are in this guide Guide: Coronavirus Community Help and Support 
When you call, you will be asked for 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.
You can also try saying "Upgrade" or "Fraud" to be connected to an advisor.
8.00am is the best time to call if you can.