1. Boost Infinite

A relatively small player on the wireless scene, Boost Mobile has a Smartphone Membership Plan that features an iPhone 15 for free with a trade-in (iPhone 11 or newer) and purchase of the carrier's $60/month unlimited plan. Crucially, though, Boost Infinite spreads the cost of the phone out over 36 months in the form of bill credits, meaning it's basically free as long as you're on the service.

2. AT&T

It also run a lot of deals that are comparable to this, especially on their Unlimited Plan plans. For instance, AT&T has had a deal for a free iPhone 15 when trading in an older iPhone (like an iPhone 12 or newer) and activating an unlimited plan (typically AT&T Unlimited Premium). The discount is in the form of monthly credits over 36 months, so you will need to maintain an active plan to benefit from the full discount.

3. Verizon

Verizon's Unlimited Ultimate Plan is a similar proposition, activate an eligible device (like an iPhone 13 or later) with the carrier, and it'll apply credits over 36 months that pay off an iPhone 15 or an iPhone 15 Pro. You will have to be on Verizon's premium unlimited plan, which costs about $90 per month for a single line.

4. T-Mobile

In the meantime, T-Mobile's Go5G Plan offers (Go5G Next and Go5G Plus) regularly include free iPhone deals with trade-ins. For example, you could trade in an iPhone 11 or newer on a Go5G Plus plan and receive an iPhone 15 at no additional cost, through bill credits over 24 months. The one advantage in T-Mobile's column, if you're on its Go5G Next plan, is that you can occasionally upgrade after just 12 months.