Scams are usually emails, texts or calls. @Kim7 said she had a letter.
However if you thisnk it's a scam then follow this advice
Reporting a scam email, text or call
Some scams might pretend to be from O2, or from an organisation you already deal with. It's important that we see examples of phishing emails, texts and websites so we can investigate and shut down scammers. To report a suspicious email, text or website:
Veritas Numquam Perit
Hi me and my husband recieved letters today saying we owe money on 2 different numbers that we have never seen. When you ring the numbers on the letters they go straight to line busy and cut off. If we loom at the letters there are little discrepancies like the dates formats are different, the names don't have capital letters compared to other o2 correspondence, etc. I was waiting g for a hour to get through to a human to talk to via phone but no one answered. And no one was available via chat. Will try again tomorrow. Don't panic it just seems like a scam. So definitely do not pay unless you get through to o2.
Welcome to the community.
Probably a scam please be careful.
Some information here
O2 can be contacted on social media if suits you
Or you can call them on 0800 081 0255