Random Extra Contract - Can't See it in my Account

Hi everyone,

I received an email saying “Welcome to O2, here’s your new tariff” and the phone number listed is not a number I recognise. I went to my local O2 shop and the worker said she couldn’t find that number on anyone’s account, and it wasn’t showing on my account, so I didn’t need to worry about it. I then later received an email from a lawyer saying I owe them around 400 Euro for this random account. I don’t think it’s fraud or a scam, but you never know. I again couldn’t find this number in my contracts, though emails about it from O2 go to my same email as what I use to log in. I’ve tried to log in with the phone number but can’t receive texts to it obviously, and even if I reset the password to my account with that number as part of my login, it still takes me to my usual account page where this random phone is not listed anywhere! I don’t have 400 Euro laying around for possible fraud or a mistake in the O2 billing, etc. I didn’t want another contract and I haven’t used, not am I able to use, this one. I can’t find a way to cancel it, nobody seems to know it exists except these lawyers...I don’t know what to do!