Andrew Hoyle/CNET

If Sprint's merger with T-Mobile goes through -- not a guarantee as the companies head to court to battle several state attorneys general who are suing to block the deal -- legacy Sprint customers should plan to give up their 4G phones. While T-Mobile has long talked about its desire to migrate customers over, it now says it will use 5G as an incentive to get people to switch.

"If you think of the new T-Mobile coming together, we have to migrate customers off legacy CDMA and get them onto handsets that are compatible with the new T-Mobile," Mark McDiarmid, T-Mobile's senior vice president of radio network engineering and development, said in an interview. "And the best way to do that is to give them 5G handsets. Or to encourage them to upgrade them."

T-Mobile has reason to get people to switch over as quick as possible. While Sprint owns a valuable swath of spectrum -- the radio signals that are the life blood of wireless service -- it utilizes a different technology for its voice network called CDMA that's incompatible with T-Mobile's GSM-based network. By getting people off the network, T-Mobile can quickly shut down the CDMA parts and move it over to be used for 5G.

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That's why T-Mobile has been so eager to get the Sprint deal done. Beyond inheriting a base of tens of millions of customers, the nation's third-largest carrier would also get Sprint's valuable spectrum, which would greatly enhance its ability to roll out a comprehensive 5G network. T-Mobile has promised price locks, free service to first responders, home connectivity to low-income families and a new $15 phone plan if its deal goes through.

The new T-Mobile will offer an incentive program to ease the transition, McDiarmid said, though there are no formalized plans yet. He noted that the move would be similar to T-Mobile's transition of MetroPCS customers when the carrier bought the network in 2012. Like Sprint, MetroPCS also used CDMA technology.

"The essence of it is that to migrate the customers to compatible handsets, the obvious place to go is to 5G because it's future-proof," McDiarmid said.

Sprint customers wouldn't have to relinquish their phones immediately. It would take three years to migrate Sprint and T-Mobile's existing networks, pending approval. During that time, many Sprint customers would already trade up their older phones for a new model.

The vast majority of midrange and high-end devices are expected to support 5G networks by then, leaving few holdouts of budget 4G handsets and "dumb" phones to move over.