> someone has used that (along with a local area code) as the phone number for a store loyalty card.

Usually because for far too long, noisy retailers wanted a "phone number" upon checkout (even if one was paying cash -- Radio Shack was an especially bad one back in the day). For those who didn't want to get yet more telemarketing calls, repeating "Jenny's number" [1] from the song was a way to "just buy" whatever it was you wanted. The minimum wage cashier didn't care, but the cash register demanded "a number". So giving the cashier Jenny's number worked.

This has largely faded now that they can track everyone via one's credit card numbers.

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