Sprint T-Mobile merger has been a mess. T-Mobile is now shutting down Sprint’s 5G network in light of it’s own. So basically, Sprint’s current 5G implementation is technically incompatible with T-Mobile’s. Now, after the merger, Sprint’s valuable 2.5GHz licenses may no longer be of value to T-Mobile. However, there’s some good news for the the existing Sprint users. T-Mobile SIM cards now work with Sprint locked smartphones. Not just that, but Sprint locked and non-paid off phones can now work with all the T-Mobile networks including Mint Mobile and Metro By T-Mobile.
Reddit user u/ALPhAWolf33 recently tested this on an iPhone 7 and an iPhone 6s; both accepted the T-Mobile networks. This is true for all the T-Mobile networks including Mint Mobile, Metro By T-Mobile. Another users u/Chaad420 also tested the same on a Sprint iPhone XR and inserted a T-Mobile SIM and it worked. Simply get a T-Mobile SIM and insert it into your phone.
However, this may only be true in terms of the 4G LTE spectrum. As for the the 5G network, you may still need to switch to a different phone compatible with T-Mobile’s 5G network. So Samsung Galaxy S10 5G users on Sprint carrier may need to upgrade to a Samsung Galaxy S20 in order to use T-Mobile’s 5G. But there are some special offers involve in order to compensate for the inconvenience.