Today I experienced the same problem as the OP above, my O2 Classic PAYG sim is showing as "not registered on network".
This is with a 2nd handset, a Samsung Galaxy S7, which I use mainly as a SatNav device in my car, because of this it requires network access very infrequently and the O2 Classic PAYG 3-2-1 setup is very suitable.
The sim was activated on 27/05/23 when I topped up with £20 credit, this was confirmed by O2 texts, the last text received was a "spam" do you need to top up text on 08/11/23.
I tried restarting, removing and replacing sim.
I called O2 customer services and was passed to payg dept. who eventually advised sim would be reactivated after I confirmed SSN, he also confirmed the sim had £19.14p credit, then call dropped.
I called back and gave landline in case of a dropped call again, I reconfirmed the details as in the first call.
The advisor said that the Classic PAYG 3-2-1 sims were discontinued, I advised my main phone was still on the Classic PAYG 3-2-1 sim and that the O2 website confirms this with my number and credit amount:-
Classic Pay As You Go
3p a minute, 2p a text, 1p a MB of 2G/3G/4G data
The advisor put me on silent hold while he looked at the issue with a Team Leader.
The advisor came back and said that the sim should be reactivated within 24 hours and that if I call back and confirm this the £19.14p credit can be reapplied to the sim.
I wish O2 would reinstate the Classic PAYG 3-2-1 option as I, like many others I'm sure, find it a cost efective and convenient option.
I do resent O2 trying to force me onto a rolling PAYG monthly payment which is more expensive and not suitable for my needs.
Come on O2 listen to your customers and provide what works for them not the other way around!
I will add an update once things have hopefully been sorted out.
