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It was like when t mobile bought metro pcs. Metro was a cdma carrier and t mobile a gsm carrier. They kept metro’s network up for a while and gradually migrated their customers over to gsm phones. From what I hear sprint’s iPhones when unlocked are universal phones so that would be an easy fix as they could just unlock the iPhones so they work on a gsm network.
 
Both networks will most likely remain live until they can transition over to full GSM.
 
If you have the TMobile version of the 7, 8, or X, you'll have TMobile voice coverage, but TMobile+Sprint LTE coverage. The Sprint version will give you TMobile+Sprint coverage for everything.
 
It will take years to merge the networks. I remember when sprint bought nextel, and they kept the networks seperate for many many years - far too long in all honesty but I mention that just to point out, a merger in many respects is not a simple flip of a switch.
 
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