@Loopyloo76 If you don't pay what you owe, you get disconnected. That isn't something that's particular to O2. I'm certain every network would do the same. You will need to pay off the remainder of your contract to cancel. If you don't, you will have a negative rating put on your credit file and you'll find yourself not being able to get a contract with any mobile provider, or be able to get credit for anything you might want to purchase.
If you want to discuss this, you need to speak to the Payment Management Team on 0800 902 0217.
The opening times are:
Monday to Friday – 8am to 8pm
Saturday – 8am to 6pm
Sunday – 9am to 6pm
Best chance of getting through is calling at 8 am