Not receiving option to pay in installments

Hello,

I just added a new line to my AT&T account for my girlfriend (switched from Verizon) yesterday, and now we want to upgrade her phone. It’s an iPhone 7 that’s been completely paid off for years now.

When I use the My AT&T app to try and upgrade her device, I’m only receiving the option to pay for the full amount up-front. Yesterday I was getting the option to pay in installments but I was unable to check out, someone at an AT&T store said it was because I had to wait 24 hours after the line was added to upgrade the phone.

Why wouldn’t I be able to pay in installments? My other line is currently set up to be paid off this way, so I don’t see why there would be an exception. Thanks.