867-5309 is from a popular song in the 80's. The song is Jenny, by Tommy Twotone. The lyrics are about seeing some "For a good time, call Jenny 867-5309" graffiti on a dive bar bathroom stall. The singer, being drunk and lonely at the bar, decides to call the number on a whim. Since the song was popular and lists the digits as part of the chorus, it is easy to remember... Memes were a thing, even in the 80's before the internet. And basically any GenX person can recite the number, even decades later.

Which means it is commonly used as a fake number wherever you can make a loyalty account with only a phone number. So if the place has a loyalty/rewards system, the popular fake number probably has real rewards to be redeemed. I can usually get a few cents per gallon off at the gas station, for instance. Because with dozens of people using the same phone number all over town, the fake accounts quickly rack up loyalty rewards.

Sometimes called the Jenny Discount by those who use the number. Worst case scenario, you give the store a meme number and nothing happens.

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?