T-Mobile and Sprint are going head to head with the new phone trade-in programs that offer the best value for used smartphones.

The ongoing rivalry between the third and fourth largest wireless carriers in the Unites States continues with a new trade-in program. Sprint has challenged T-Mobile and other major carriers including AT&T and Verizon in offering the best trade-in value for used smartphones. The carrier is offering an instant in-store credit up to $300 for eligible phones.

Sprint is out bidding T-Mobile by offering trade-ins for up to three phones per line and five devices in one calendar year.

T-Mobile was the first to announce its trade-in program on Monday, where it accepts one phone per line and applies the credit towards the purchase of a new phone. The carrier, however, is also offering to top the best offer in the market  with an additional $50 credit on the phone's value. Sprint highlights its own strengths, which lies in multiple trade-ins and application of the credit towards purchase of a new phone, accessories or monthly bill.

"Our trade-in program is well known in the industry and when we guarantee the best value over any of our competitors, we're taking this to another level," David Owens, Sprint's senior vice president-Product, said in an official press release, Monday. "Offering a competitive price match was the next logical step to ensure existing and new customers understood that we make our trade-in program a top priority and are willing to provide the very best value in the industry. We want to make it easy for our sales reps so they never have to worry."

The ongoing competition between Sprint and T-Mobile is good for end users, who benefit from the offers.

The new trade-in program comes ahead of the most-anticipated iPhone 6 release. New flagship product release means more customers are willing to upgrade to the newer handsets and trade-in offers are the best shot at doing so.

"We're going to see some of the most phenomenal devices ever in the coming months, and that means a whole lot of Americans trading-in their devices and upgrading," John Legere, president and CEO of T-Mobile, said in a press release. "For years, the big carriers have been ripping off their customers with low-ball trade-in values, so we're putting an end to that and guaranteeing we'll give customers what they deserve - the best value in the industry."

Customers can get the value of the phone by walking into the carrier stores or explore options online with T-Mobile or Sprint.