When I lived in Uptown I could typically get one just by standing on Broadway for a little while. When I lived in Logan Square I'd call and they'd be there in 20 minutes. This is well before either Uptown or Logan Square were particularly gentrified.
I can believe if you were going to/from the suburbs it may have been harder. The cabbies could know your destination and could just ignore the dispatcher if they didn't want to run the trip.