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It's other cab drivers for the most part, sometimes friends and family. Driving around all day is extremely boring - as much as cell phone use sucks in general while driving, they do what they do to stay sane.

Besides the obvious boredom-killing, I've also heard of it being used as a safety feature. It wasn't that long ago that drivers would be robbed or assaulted (heck, an UberX driver here was just robbed a few weeks ago).

I had a parking lot attendant at work who for years was always on his phone when I arrived. One day I asked him who he always talks to. He says he has another business besides the parking lot. So maybe at least some of these cabs are having business calls as well. If you can hustle to be a cab driver, you can hustle in other ways as well at the same time.

Some people are absurdly chatty on the phone. I don't understand it myself, but different strokes, etc.

You could outsource your customer service to taxi drivers