Harassed by ADT security & Norman Bisset and Associates

I'm being harassed by this security company. I had their service once and I cancelled it because their response was very slow to incidents and now they claim that I owe them. Apparently there's a collection company that bought my account from them so whenever I call them to ask them to explain to me why I owe them they transfer me to this collection company. The collection company's response is that they don't know why I owe ADT because they just bought the account from them. This is stupid.

There's now this law firm called Norman Bisset and Associates that keeps sending me final demand letters for ADT, that I can't even open because they are encrypted. It's a one way conversation with this firm by the way. They send me emails but when I respond I get no answer.