The worst experience I had with this involved 4 customer service representatives simply hanging up (after a 30+ minute wait) after exhausting their script. I simply recorded the final call, stopping paying the bill, and sent the documentation I had to the credit reference agency when the contract went into default
At some point they sent a debt collection agency, that was much less stressful than it sounds. They called me up, "you owe $telco money", "No I don't", "oh?", "Yes, I have complete documentation of cancelling it, but their CS reps kept hanging up. Sorry, you've been had.", "Oh, this again. Sorry to have bothered you.", never heard from the debt collection agency again.
At some point they sent a debt collection agency, that was much less stressful than it sounds. They called me up, "you owe $telco money", "No I don't", "oh?", "Yes, I have complete documentation of cancelling it, but their CS reps kept hanging up. Sorry, you've been had.", "Oh, this again. Sorry to have bothered you.", never heard from the debt collection agency again.