Praise be to Allah, the Lord of the Worlds; may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Calling a cab is a form of fair wage, not leased property. Concerning fair wage, it is permissible for the hireling (Taxi driver) to offer his services to several beneficiaries at the same time. According to Jurists, here, the driver is called "Joint hireling." Accordingly, it is permissible for the cab driver to pick up other passengers on his way to pick the passenger that called him first. However, this is provided that the driver doesn`t charge more money if he has to change his way to pick up the new passenger to his destination and that he starts the meter once he picks up the first passenger, not before. And Allah the Almighty Knows Best.