Dont pay it , they done the same to me
i never payed them a penny and they passed it on to 4 debt collection compaines who all passed it back to 3 saying i didnt have to pay as i had valid reasons as they had contiued to charge me even though my 12 months contract was up and the phone had been cut off.
if you think you owe the last months bill , then just pay that. They wont take you to court , just a threat.