The basic ordering process is almost identical at all four major carriers. Preorders start Friday and the new iPhone 7 models reach customers a week later on Sept. 16. And most customers will have to pay full freight for their iPhones, either all at once or in monthly installments, as the carriers have moved away from subsidizing the purchase price in return for a two-year contract. But the carriers have often used the lure of new iPhones as an opportunity to entice customers to switch providers. This year, all four are offering a free iPhone 7 with 32 Gb, the entry level model, for free with a trade in of an older iPhone 6 or 6S phone.

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T-Mobile, the third-largest carrier, kicked off the trade in promo trend, saying on Thursday that it would let new or existing customers who trade in either of the last two iPhone models, the 6 or 6S, have a free iPhone 7 with 32 GB. Customers who want a more expensive model pay the difference in price from the 32 GB model.

T-Mobile (TMUS) said the trade-in iPhone must be in working condition without water damage or a cracked screen. And customers who leave T-Mobile within two years of getting the free iPhone must either turn in the phone or pay a proportionate share of the original price.

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Sprint (S) announced a similar trade in plan later on Thursday. New or current customers can get a new iPhone 7 32 GB or equivalent credit if they turn in a working iPhone 6 or 6S and sign up for one of the carrier’s current rate plans. Sprint is also accepting trade ins of the Samsung Galaxy S7 and S7 edge. And a similar two year restriction applies.

Sprint also said it would give a customer buying a new 128 GB model iPhone 7 online a free upgrade to the 256 GB model.

Verizon Communications (VZ), the largest carrier, will also take trade ins, it announced on Thursday night. Customers with an iPhone 6 or 6S model can get an iPhone 7 free. Customers with an older iPhone 5, 5c, 5S, or SE can get a $400 credit towards a new device. And Android switchers can get $300 on a trade in. The same kind of two year restriction applies.

AT&T (T) initially offered only a limited its deal to customers of its two pay TV services, DirecTV and U-verse. Such customers who switch a wireless line from another carrier will be entitled to a free iPhone 7 32 GB or credit equal to the value of that phone on a more expensive model. But by midday on Friday, after pre-orders had commenced, AT&T also jumped on the trade in band wagon, offering similar terms as the other carriers.

(Update: This story was updated at 2:35 p.m. on September 8, 2016 to add Sprint’s iPhone promotion, at 8:15 a.m. on September 9 to add Verizon’s and at 1:30 p.m. on September 9 to add AT&T’s trade in offer.)