I'm not sure if I'm answering the right question or not, kevjen888. We had two IUP accounts (each with one of our two different phone numbers) and two phones bought on IUP in November 2019. In September 2020, we preordered with the intention of sending the two phones back. In the interim 14 days you have to send them back, we decided to keep them instead because the balance was low enough that it seemed insane to give it back and let them make money on it vs. passing it on to family members who otherwise wouldn't buy a new phone at MSRP. From that point forward, Citizens just kept taking the monthly payment on the November 2019 phones and loan numbers, and then we had two new phones with new loan numbers where we also paid monthly, thus having four outstanding loans. (I didn't know how they'd handle it at the time, either, and was prepared for them to send me a bill when we didn't send the trade-in and just pay it off, but if they were going to continue to allow monthly payments at 0%, all the better.)
We're down to just two loans now because I paid the November 2019 balances off, and now we're set to upgrade the other two lines during this 2021 preorder. At that point, I guess we choose again if we want to send them back, and it makes more sense to this time since only 12 payments were paid instead of the 14 payments paid last year due to the late rollout of the iPhone 12. If they don't get the trade-in back in 14 days, they assume you are keeping it and just keep billing you. At least that is how it has worked for people in the past. I'm sure they have a limit to how many of these outstanding loans they would allow before they would say no to a new one, which is why I made sure to pay off the balance of the 2019 loans before I set up the IUP on the coming 2021 phones.
When signing up for the IUP, you have to use an individual loan for each phone you purchase. You give them your name, social security number, main number the account is tied to, and then they want to know what line it will be used on. We have two of those loans, same social security number and account holder used, but there were two different phone lines and two different loans, although they tied back to the original main number on our AT&T account. I hope that makes some kind of sense. I know it's confusing! Sorry this is so long.
EDIT: I just realized the dates above are incorrect. Phones came out in September 2019 and then in November 2020. Even I'm confused at this point! 🤣