The phone's cost is reimbursed over 24 months. If you cancel your service early, you immediately owe the full remaining balance of the phone.
If you are coming from another carrier, the value is hard to beat. T-Mobile is currently offering up to $830–$1,100 off the iPhone 17 series via 24 monthly bill credits. On their premium Experience Beyond plan, you can often get the device for "free" without a trade-in.
Expect to pay the sales tax on the full retail price of both phones plus a $35 connection fee per line at checkout.