Starting Monday, new and existing Sprint customers can get an iPhone 8 if they trade in an iPhone 7, Samsung Galaxy S8 or Samsung Note 8.

Those owning older iPhones, and recent models from Samsung, Motorola, Google and LG Electronics can qualify for half-off prices on the iPhone 8, Sprint says.

With its move, Sprint is challenging its larger peers to follow suit or face subscriber losses with bargain-hunting customers.

Last week, AT&T started the battle by offering a buy-one-get-one-free iPhone deal for customers who also subscribe to its DirecTV satellite service.

AT&T's promotion started Friday and applies to the iPhone 8 and 8 Plus, among other phones. In addition, AT&T customers who purchase an iPhone can also get an iPad for $99.99. The deal requires a two-year wireless service contract.

Neither Sprint nor At&T have said whether their deals will be extended to Apple's most expensive phone ever -- the $999 iPhone X, aka 10.

T-Mobile offers a $300 trade-in credit for customers buying the iPhone X, as well as the new 8 and 8 Plus. The trade-in must be an iPhone 6 or newer.

Verizon is offering a $300 credit on the iPhone 8 and 8 Plus with trade-in. The company's list price for the iPhone 8 is $699.99 or $29.17 a month for 24 months, and $799.99 for the 8 Plus or 24 monthly payments of $33.33.

All the U.S. carriers also sell the connected Apple Watch. The device needs a $10-a-month data plan. As part of the deals, carriers are giving away the first three months of wireless service.