Phone trade in.

Nope.

In 3 months, the credits for your trade-in phone should begin and you should get credit for 3 months at $27.78 each or a total of $83.34 that means out of the original 1200, $83.34 will have been paid. You will still owe over $1,100 on the phone.

You did not read how the trade-in deal works. The trade-in deal does not give you a lump sum credit. It provides bill credits over 36 months.  

This means every month AT&T will apply the installment to your bill of $33.33 and they will also put a corresponding credit of $27.78 on your bill..

Once the credits begin you can pay off and unlock your phone if you wish to do so. But if you cancel the line or upgrade the line the credits will stop

A little over a year ago I traded in my galaxy s9 as credit for the 22 ultra, the s22 is on the 36 months thing, I want to get the iPhone 15 pro max and was wondering if I can use my phone that is on the trade in plan as some sort of trade in for the 15

@Trashyll The phone you trade in for the 15 has to be fully paid off.  You’ll have to pay off the s22 to trade it in.

how do i pay it off if its on the 36 months thing

You go into your account online and pay off the balance due in full.

when i traded in my s9 they said it would count for 800 off the 1200 so does that mean i will have to pay 400

No, that’s not how it works.  If you bought a $1200 phone on a 36 month plan, it’s $33.33 a month installment payment.  If you’ve been paying it for 12 months, you’ve paid off $400 of the $1200, so you would need to pay $800 to pay it off.  The offsetting monthly credits don’t reduce the amount to pay it off, they just effectively reduce your monthly payment.  Also, if you’re using the trade to do a similar promotion trade for a new phone, you’ll lose your remaining credits on the old phone.  You can’t keep getting monthly credits on the old phone you traded in, as well as on the new phone you traded for.  You can’t double dip.