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The thing to keep in mind here is that each such pizzeria had a specific territory it staked out. There was an effective radius from every location, and the drivers were often very experienced with that chunk of town. I also recall wall-mounted maps near the phone so they could easily tell the customer to call a closer Domino's or Pizza Hut over if they were out of range. So after a while, you just learn the region, memorize the street names, and off you go. Finding a house number was the only real risk.
Advertising was also typically done door-to-door with flyers and fridge magnets, along with phone numbers for YOUR local franchise. As a franchise owner you'd have your family or hire some kids to canvas every so often. I suppose that helped with any confusion, but there was nothing keeping you from getting a hold of the wrong number from the phone book or a friend.
With GPS navigation everywhere, I'm betting that drivers can range further than ever before. The calculus is probably more like "google says you're 40 minutes out right now, so no", than "you're not one of our customers."
I was one of those summer "crew" kids! It was fun, easy work at the time. Always wanted to move up to phones, but there weren't openings. (bear in mind, it was my first year of HS, my goals were low)