ADT contacts me 19 months later via agency

I cancelled my services with ADT in Jan 2014 and was willing to pay for services rendered up until this point. However ADT did not return any of my calls and even my visits to ADT N1 City at the time. Then in May 2014 they called me and wanted their money. I made it clear that I cancelled and showed them proof of this. The responsible person at the time said that if I sent this through all would be well and she would sent me more information. They never followed up again after my email in June 2014 to show the proof of cancellation. Thus it has now lapse and all of sudden 17 months later in Sept 2015 I get a sms to say, please pay back the money now. Upon me calling the number displayed, I get a very annoyed person who says, just pay back the money.<br> ADT contact me, you still have not even collected your equipment in my roof.