Lots of places have Reward Points for handing over your phone number. I've used Jenny's number for years to get discounted gas. Usually only $0.03 but one time I got $0.40 off per gallon.. been chasing that dragon ever since, lol.

So, after that song rose in popularity, phone companies de-registered it entirely. Because it's a number that no-one has, my guess is that some stores/businesses use it as shorthand to put a discount under for something like employee discount or for "we messed up, here's a discount" discounts.

Because so many people use it that it racks up tons of loyalty points.

Companies want you to associate your transactions with a phone number so they have data on who is buying what. For instance if you go to Safeway it's common that the price of ice cream will be twice as expensive if you haven't registered a phone number with them. But you can give someone else's phone number that has already been registered and that will also work, or register a fake number, a number lots of people would have thought of is likely to have already been registered. At least that was how it worked last time I checked, that was a long time ago.

Oooh, not the store you're giving the number to, but you can buy that number from a carrier as a dummy number for a discount because no one wants it due to prank calls?

https://en.wikipedia.org/wiki/867-5309/Jenny