No Agreement but handed over for Debt Collection

For over a year ADT security has been handing our "account" over to numerous debt collection agencies. We have been asking for the so-called signed contract as we have no agreement with ADT at all. We are then told that there is a signed contract and they will send it, but it never happens. They are harassing us from as early as 6:00 AM in the mornings, throughout the whole day and even as late as 10:00 PM at night! Come Saturdays and even Sundays, they continuously harass us. I am out of ideas as to how to get them to stop. Is it not ********** to hand someone over and state they have a signed agreement, but when they ask you for this so-called signed contract they never send it and then just hand you over to the next debt collection company?