Is it permissible for the company to charge additional fees for receiving calls, knowing that I subscribed to a service that receives calls throughout the month for a sum I paid, and would I be sinful if I do not pay the additional amount resulting from that, given that I will not use the line again?

The deciding factor regarding the company's entitlement to what it demands is the contract agreed upon between you. If the contract stipulates its entitlement to the value of receiving calls, then you must pay it. Otherwise, it has no right to compel you to do so. The contract is the law between the contracting parties.