Is it permissible to pay a fixed monthly amount to the telecommunications company for phone service, even if the service is not used during the month, and is it permissible to rent this phone to others for money?

If you lease a mobile phone line from a telecommunications company for a specified fee and a known duration, this contract is valid due to the fulfillment of the conditions of ijara (leasing/renting). The variation in the number of calls does not affect the validity of the contract, as the lessor has enabled you to benefit from it. Accordingly, you are permitted to lease the phone to whomever you wish, because you own the usufruct and have the right to dispose of it.