Trying to cancel contract without an active handset

I have been trying to cancel my contract since February, when my daughter moved to a new handset and different phone number. The ID number has no active handset so each time I have tried the links to cancel the account it sends a text into a black hole, and there is no phone line I can call to deal with this! at the time of buying new phone, carphone warehouse told me they were cancelling the ID account and contract (which had already run it’s two years). I am now getting debt collection letters every few days and threats that the phone is restricted - but the phone doesn’t exist! I am paying the bills as they come but have no idea how to get out of this situation?