Give the dispatcher your current location, where you want to go to. He allocates a cab to come and pick you up, drop you off. If you want to get fancier with gps, fine. But nothing beats the flexibility (and in several downtown locations, the speed and convenience) of hailing a cab, and knowing that you're getting a standardized metered rate.

At least in India, there isn't a cap on the number of medallions that can be issued to cabbies. And there's a substantial amount of competition from private radio cab cab operators, who have an association that fixes the price - which typically keeps in sync with "regular" cabs (painted black with yellow tops) thanks to market forces.