I have stumbled upon this billing issue in past from PTCL and I only had to pay one month's bill after disconnecting my line. As it turns out the PTCL bill cycle is like this. You get charged for services after a month. So in case you have terminated your connection in the middle of the current month, you will be paying for the last month's bill so there will be one more payment left to be made in respect of current month's bill. This how it's always been.