trade in installments of 36 months when you buy new iPhone from Apple after 12 months

It wouldn't be the first time AT&t gave bad advice. 

Just do not upgrade with AT&t installments. You purchase an unlocked phone from Apple at full cost. Then you switch your service to the new phone.  

Stop listening to AT&t support. 

It's their job to make sure you're continue to be entirely dependent on the service provider.  

what happens to trade in installments of 36 months when you buy new iPhone from Apple after 12 months and keep the same line with AT&T and do not take any upgrade promotion from AT&T? 

It doesn't matter whether you take an upgrade promotion or not. Anytime you upgrade your line your previous installment loan is terminated, and any promotion that might have been included would also be void.

Did you upgrade through Apple with AT&t financing?  Yes?  

You cannot have two installment agreements on one phone line. So anytime you enter into a new installment agreement the previous one must either be paid off, or as a Next Up early upgrade you must return the previous phone to AT&t to have the remainder of your installments waived (this only works if you have next up early upgrade option)

Hopefully reading the agreement you entered into will help explain.  Sample below

If you upgraded through Apple and chose AT&t financing, you have breached your part of the 36 month installment agreement.  This means you have a new installment agreement, you have to either pay off or return your previous phone, and the deal you had on a previous trade-in is now void.

If you purchased a phone from Apple (or anywhere else),  without AT&t financing it has no effect as long as you continue to pay for AT&t service and the installment, and receive credits against that previous trade-in.

I have paid up iPhone 14 all installments which I got from Next Up as I wanted to unlock the phone for international travel. But still pays me in 36 installments towards trade in value of my previous phone iPhone 13. As I change iPhone every year. I have been loosing trade in value in installments after 12 months. To avoid that I will not be taking any shady promotion from AT&T but next year I will buy new iPhone from directly from Apple. My question is that in that case I will be loosing remaining trade in value in 36 months installments? I was given conflicting answers from AT&T customer service. My next step will be to write AT&T legal department to get confirmed answer as next time I do not want to loose my trade in value of my previous phone which I gave AT&T for trade in for the amount of $800. If AT&T keep on my taking my paid up iPhone for $335 which I could easily sell outside for $800. Please let me have confirmed answer from AT&T.

But still pays me in 36 installments  credits towards trade in value of my previous phone iPhone 13. As I change  upgrading with att installment loan iPhone every year. I have been loosing trade in value in installments after 12 months.

    um...yeah.  read your loan agreement.

The promotion through AT&t is the same offered through Verizon and T-Mobile.  If you upgrade your line, your previous installment and previous phone promotion terminates.  

It's what you agreed to when you signed up.

If you wanted to work around all you had to do was ask.

    A) You could be doing your trade in directly with Apple every year and get a lump sum toward your new phone. 

   B) You could be upgrading with carrier deals every other year, and Apple on the alternate years.

It is not a scam or shady just because you don't understand how it works.  And it works exactly the way it is written in the agreement that you signed.

🤦‍♂️

Iphone 14 was upgraded with AT&T and traded in iPhone 13 in January 2023. But I have paid up all money towards iPhone 14 to unlock the phone as I travelled often internationally and I need to have unlocked phone. But AT&T still owes me monthly payment towards traded iPhone 13 in January 2023. My only concerns is that what happens next year in 2024 if I buy iPhone 15 from Apple with all cash and carry on same line which I have with AT&T, in that case will I loose remaining monthly trade in credit on my AT&T bill towards traded in my iPhone 13 in January 2023? And in 2024 I will not trade my iPhone 14 with AT&T or/and I will not take any promotion from AT&T in year 2024 or in future. Again I received conflicting answer from different call to customer service of AT&T.

Read my replies, I already answered your question.

Thanks. I read your answer but AT&T customer service giving me answer that even if I buy from third party like Apple with full payment,  AT&T still considers as upgrade and AT&T will stop paying remaining trade in  value credit to my bill. I appreciate your response but I still need to reach out department of AT&T in writing. I like to be very very careful with AT&T.

Thanks again for your quick response. Yes, I learned & experience with AT&T never to trade in any phone with AT&T and do not take promotion or upgrade from AT&T. It is much better to sell iPhone outside than to trade in with AT&T. I will not make that mistake again.

Let's assume a case that: 
While purchasing an iPhone 12(1) & trading in an iPhone 11 through AT&T's promotion since Sepember 2020, a customer unluckily got it stolen in January 2021. Then, the customer bought out a new iPhone 12(2) direclty from Apple, still keeping the same number and service by AT&T. If this is the case, AT&T still requests the monthly installment amount for the iPhone 12(1) and also credits monthly for the iPhone 11, both of which offset each other(usually). 

If OP's concern happens to be true, AT&T would not credit towards the iPhone 11 after January 2021 since the customer "upgraded" with the iPhone 12(2). However, it's nonsens. Therefore, AT&T does not forfeit/cancel the installment & trade-in agreements when a customer buys out a new Phone replacing the installmented phone as long as the customer keeps the same numbers and service. Sometimes, unwanted event happens, and a customer should be able to replace with an old phone he/she has kept for spare, or rather buy a new one for peace of mind.