A:AnswerThat one-time payment option you're referring to is only applicable to new customers or new lines. It's a bit misleading, I know. The upgrade price is full MSRP.
A:AnswerIf I understand your question the monthly payment is for the cost of the phone only. The cost of ultimate talk, text and data would depend on the plan and carrier you chose, I think, hope I was a able to help you
A:AnswerWell they have a chart showing what you may be able to get.
https://www.bestbuy.com/site/conditional-offers/att-iphone-12-trade-in/pcmcat1602770983908.c?id=pcmcat1602770983908
Thats the AT&T one which shows you for the trade in and models.