T-Mobile USA will this week become the last of the “big four” US operators to launch the iPhone, announcing a trade-in deal for existing owners of Apple’s smartphone.

The company will allow owners of iPhone 4 and iPhone 4S devices to trade-up to the iPhone 5 with no upfont charges, and a $20 per month payment for 24 months.

Depending on the value of the device traded-in, customers also stand to receive a credit of up to $120 which can be used toward monthly payments, for an existing T-Mobile bill, or for accessories.

For other “well-qualified” customers, there will be a $99.99 upfront charge for the iPhone 5, followed by the monthly payments.

The trade-in deal means that existing iPhone 4/iPhone 4S customers can receive up to $220 from T-Mobile in discounts and credit – which is hardly top-dollar when compared to some handset recycling firms.

T-Mobile USA is trying to reassert its position in the US market through a strategy of splitting device costs and subsidies from its service plans.

For smartphones, this generally means an upfront charge and a monthly payment plan that is separate from its service subscription, rather than lumped-in as with the traditional contract model.

The company said that this gives more flexibility, as customers are not locked in to price packages dictated by operators, and have more freedom when it comes to trade-ins and upgrades.