FAILURE TO CANCEL PHONE LINES

I sent in a request on the 26th of February to cancel 2 of my business phone lines and it still hasn't been cancelled. I've phoned in 3 times already and all they can tell me is to email the letter again, which I did every time and still those 2 phone lines are still operational which means I'm going to have to pay for lines I haven't used which was suppose to have been closed on the 31st of March. I don't know what to do because when I phone they will make me send them the same letter I've sent 3 times already.