Accounts in a mess

I had 2 tracker systems for many years. I cancelled the one last year and the other in March this year. I had to pay to end April for the notice month. However, I now get a notice from a debt collector MCS saying I owe Tracker. I call them and discover they just kept invoicing me. They reversed these and sent me a mail to say my account was at zero balance. However, MCS is still chasing and threatening me.