Then, have it so that when anyone calls 383-CHEX, the moo rings the phones in the taxis (all of them) two or three times. If no one answers, you get the usual automated thing.
This way, a player who want's a taxi ride will always be connected to a player cabbie if one is in their car, not AFK and ready to take on a fare. The player cabbie can always chose to ignore the call and let it go to the automated system.
And if a cabbie is answering calls but taking forever to get to the customer (because they are trying to snatch all the fares by answering first and end up with a deep queue of people waiting on rides), then the customers should complain and the cabie face consequences.
Not sure if this is a good idea or not but it popped in my head after seeing someone asking "Any cabs around?" for the hundredth time on SIC. And it would really push biz to players over NPCs.