I don't know if I'd call Waffle House a "ma and pa" operation. Every Waffle House I've been to seems to use the same POTS phone line that is also their business phone, for dial-up for credit card validation.
And most convenience stores I've been to also seem to share their regular voice phone line for dial-up card validation. And these stores seem to be part of a "chain", and sell "name brand" gasoline. BTW, I'm referring to the card validation in the store at the checkout counter. I seem to think that the card readers at the gas pumps are somehow connected by dedicated links.
There's always some other employee trying to make personal phone calls or otherwise talking on their business lines, when I'm trying to checkout and pay with a card, when at the counter in convenience stores or at a Waffle House. I also have found the same thing at some other "chain" fast food places as well.
a/b