In 2005, Sprint bought Nextel. A network that used an entirely different technology than its own network. It took the company nearly eight years to actually be able to use that spectrum from the Nextel purchase (partly because the combined company had to keep Nextel's iDEN network up and running, because of a government mandate). For many, those memories are coming back now with a Sprint and T-Mobile merger looking to take place. Simply because both carriers use different technologies - T-Mobile using GSM and Sprint on CDMA, though that will change once both go all-in on LTE and get rid of their legacy voice networks. Thus Sprint and T-Mobile are vowing that this merger won't bring flashbacks of the Sprint and Nextel merger from over a decade ago.
The two companies have already begun taking several steps to prepare for the merger, including the new roaming agreement for Sprint to roam on T-Mobile's network for the next four years. And that agreement will be in place even if the merger does break apart. T-Mobile has also said that around 20 million Sprint customers already have phones that will work on T-Mobile's network from day one. That's about half of Sprint's customer-base, so it's a much different thing than when Sprint and Nextel merged, seeing as zero of Nextel's customers worked on Sprint's network then. The reason for so many phones on Sprint's network already working with T-Mobile is because of LTE. All of the carriers are using the same LTE technology, but these phones also have unlocked GSM bands. And while those bands are there for roaming when customers travel to other countries, the bands are supported by T-Mobile and AT&T. That shows just how far the technology has come in the 13 years since Sprint acquired Nextel.
Now, there will likely be some customers that are not willing to buy a new phone that would work on T-Mobile's network, day one, but that's a much smaller number and percentage of Sprint's customers. So it won't be as hard, this time around. That is why it's a good thing that most phones these days do work on all four US carriers. Now when it comes to the CDMA network that Sprint has right now, it'll likely be a few years before T-Mobile is able to refarm that spectrum for LTE or even 5G down the road, but the spectrum that T-Mobile wants to use first is the 800MHz and 2.5GHz for low-band and for capacity.