Is it permissible, Islamically, to subscribe to a prepaid SIM card offer that grants free calls and messages in exchange for a monthly charge, with the condition that any remaining balance is not carried over to the following month?

The transaction mentioned in the question—whereby the subscriber pays a specific amount in exchange for benefiting from the company's services—is a legally permissible ijara (leasing) contract. It is a sale of usufructs, and for its validity, it is stipulated that the usufruct must be permissible, known, valuable, and obtainable, with the duration of the ijara specified. These conditions are met in the consumer's agreement with telecommunications companies.

If the agreed-upon period ends, the ijara terminates, and the subscriber must pay the full subscription fee even if they did not utilize all the features, as long as the lessor made it possible for them to benefit.