Why the “Current AT&T account type isn’t supported on Apple” error message?

I’m trying to order an iPhone 11 for my mom through AT&T payment plan. When I enter her phone #, apple gives me a message “We can proceed, but there’s something you should know.

Your current AT&T account type isn’t supported on apple.com.”

So I tried entering the other two phone #’s from that same AT&T account, and they went through.

This account used to be under business, but now it’s switched to consumers.

When it was business, I tried upgrading her phone through Apple, but turns out Apple doesn’t support business accounts. So I switched the account to consumers.

Anyways, it’s strange that the other two lines go through, but hers does not.

Why do you think the issue? Maybe because I tried upgrading her phone # back when it was under a business account? So Apple blocked specifically her #?