Cancellation fee/ outstanding premium after a year?

I cancelled ADT in April 2016. I was told I had to give 20 days notice before cancellation. Thereafter I received an email stating I was liable for May's premium. I requested further explanation but none came forth. Recently I was contacted by a debt recovery firm stating that I owe ADT for a cancellation fee which was never mentioned in the previous communication. The fee happens to be double the premium I supposedly owed for May.

ADT is merely attempting to recover cancellation losses by ******** former clients.