sold car in april still being billed

I sold my car in april and cancelled my insurance, they sent a letter to tracker to then cancel the respective tracker account, a month later I receive a call that I owe them money, so I paid the amount thinking my account would be called. to my surprise it wasn't, I receive another call a month down the line and get told no because the date that I \'closed'' my account fell after the 15th of that month I still owed them