Rain billing you after you stopped using their service

Rain is a internet services provider, I stopped paying my prepaid/postpaid product knowing that it should automatically cancel. Because if you haven't paid a day later after your agreed date they stop your service. After a few months get calls from rain and notice I owe them 3months worth of subscription this and won't be canceled until I'm paying it. Well I'm not going to pay it if I put that Sim card in a phone it does not work. This is unacceptable