Cancellation of ADT

A year back i have cancelled my ADT account and faxed cancellation twice (which i don't have proof of anymore)<br> now i receive a message stateting - Your ADT account has been handed to Debtsource Ltd for collection.<br> Firstly when i moved into our current home which had an alarm in already and i activated it after a couple of months. It was very easy opening an account with you and it was a hassle cancelling it (which i did).<br> I'm sure if you go and check on your system or listen to calls that was made to you, You will find that it was cancelled. <br> <br> Can this be sorted out promptly as i don't want it to have a negative effect on my credit report.