What is the reason for considering the landlord's condition and adherence to it, even though the tenant has acquired ownership of the usufruct? And am I obligated to change the ownership of the phone line that my father bought for me, especially since the company stipulates that it cannot be transferred to anyone other than its owner?

The majority of jurists hold that the lessee has the right to utilize the leased property himself or through his representative, as he owns the usufruct by virtue of the contract. They consider the lessor's condition that the lessee must utilize the usufruct himself to be invalid, as it contradicts the essence of the contract. However, they differed on the validity of the contract itself. The Hanafis and Hanbalis consider the contract valid and the condition void, while the Shafi'is consider the contract void. Some scholars, such as Sheikh al-Islam Ibn Taymiyyah, dissented, permitting the owner to stipulate that the lessee must utilize the benefit himself. They argue that the default rule for conditions is validity unless they contradict Islamic law, and that the lessor has a legitimate interest in this restriction, and the lessee has voluntarily accepted the condition.