Can't receive calls or send MMI codes

[PDDobster]

I can't receive calls, and when I send a MMI code (e.g. *#10# or ##002#) I get "Connection problem or invalid MMI code."
I can send calls, and I can also send/receive texts fine.
Using my previous phone also yields the same results. Same with turning off my wifi.
I don't have wifi calling as on option in "My O2", and my phone is just a standard unlocked phone. 

Any help would be appreciated!

[pgn]

PAYG and MMI codes are hit and miss, as PAYG does not support certain advanced network features, like call diversion, etc.

A lot of the stuff that needed MMI codes has now been moved to My O2 app for Contract users, eg adjust number of rings before VM answer, call diversion, etc.

Does your PAYG MyO2 let you see anything that may help? Note the App and Web versions of My O2 can differ in this respect.

Stepping down to 3G in your settings sometimes helped, but 3G may already be gone where you are. 

WiFi calling has yet to reach all O2 PAYG customers yet, not that you had it on anyway.

Have you turned off Voicemail, Guide: All you need to know about O2 Voicemail 901 ?

Also for balance queries: https://www.o2.co.uk/help/account/billing-and-usage/checking-your-pay-as-you-go-balance

What are you trying to do, exactly, @PDDobster?

[PDDobster]

I'm just trying to be able to receive calls again, and it's what I tried before posting here as those seemed to be common things to try for the issue I am having.
I haven't turned off voicemail, but I doubt that would be an issue as I haven't changed anything with voicemail for years.

[PDDobster]

Oh, I forgot to mention, I don't have barring and call diverts available.

[PDDobster]

It is entirely possible that I didn't realise that the check balance MMI code was changed/removed, and is a non issue.

[PDDobster]

Never mind, might've just been the network being weird yesterday, I can receive calls fine now.
Ty for the help regardless.

[pgn]

You're welcome, @PDDobster.

[Spindle]

The check balance code *#10# still works fine for me?

[Nat01]

I am also having the exact same issue at the moment. I was told I was migrated to a new PAYG system on the exact day that I lost service and this all started happening. O2 are saying that the SIM card is the problem when the SIM is working fine, but the system broke my ability to send MMI codes or have voicemail call forwarding or balance messages after every call or text sent at all.

[pgn]

Try putting the SIM in another handset to eliminate SIM issues from the mix. You can always run a dry, lint-free cloth over the contacts of the SIM, to be sure. A replacement SIM can be installed and configured for free at any O2 Shop, including balance transfer, upon presentation of the handset , SIM and valid photo id.

O2's systems usually update overnight - so check again this morning, @Nat01 

I don't think forwarding calls to anywhere other than Voicemail is an option on PAYG.

MMI codes are trickier, often dependent on which message client you are using on your handset.

Balance message after calls can be disabled or enabled by O2, contact the UK Social Media team via FB or Insta only after 7am today, links in  https://www.o2.co.uk/abouto2/social-media

Methods to check your balance
Call 4444: Dial 4444 for free from your mobile phone and follow the prompts to hear your balance.
Text message: Text BALANCE to 20202.
My O2 app: Download the My O2 app and sign in to view your credit.
My O2 website: Sign in to your account on the O2 website to see your balance.

Good luck!