The carrier will also offer up to $350 per line to T-Mo subscribers to cover costs related to switching networks.
An earlier version of this story did not account for the fact that T-Mobile is waiving the $99 down payment on iPhones when people trade in their old devices. With this factored into the overall savings of the trade-in, T-Mobile's trade-in program actually offers consumers a better value than other trade-in sites such as Nextworth.
Trade in a 16GB iPhone 6, and you can get an iPhone 6S for $1 a month or an iPhone 6s Plus for $5 a month.
Sprint and T-Mobile each announce what look like incredibly cheap pricing for the new iPhone 6S and 6S Plus. What's the catch?
This marks the lowest monthly rate for an iPhone in an increasingly competitive environment. The carrier also confirmed it would sell the Apple Watch on Friday.
The carrier's outspoken CEO John Legere touted T-Mobile's iPhone 6S preorders in a Tuesday tweet.
But there's a catch: You have to commit to a new smartphone as part of the new upgrade program. Also, only a select number of flagship phones are available for an upgrade.
The carrier's new "Never Settle" promotion will give Verizon customers a free phone to play with for 14 days and then pay Verizon's early termination fee for customers who switch over permanently.
CNET's Marguerite Reardon takes a look at T-Mobile's new plan to entice wireless customers to jump ship from their existing carriers.
The carrier is also guaranteeing that prices won't increase for customers on existing plans -- even if you're on a promotional program.