Is it permissible to gift a phone line to a sister, with the fear that she might use it for obscene talk, backbiting, or sin?

If the giver (al-Mahdi) strongly believes that the recipient (al-Muhda ilayh) will use the gift for an act of disobedience to Allah, then it is not permissible for him to give it, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." As for a mere possibility, there is no harm in the gift, and one should think well of Muslims.