I thought Sprint phones could roam on Verizon? I remember when I was on Sprint way back when, my phone would always roam to Verizon when out of Sprint coverage area. It worked great.
It depends on where they have roaming agreements. Also, Sprint and Verizon have different LTE bands. On LTE, Sprint's roaming agreements are pretty much with a small collection of regional carriers like US Cellular and C-Spire. Verizon has agreements with a different band of rural carriers.
LTE spectrum is much more fragmented than 2G or 3G spectrum ever was. There were basically 2 major bands of 2G and 4 bands of 3G. There are about a dozen bands of LTE in use in the US between all the carriers.
Verizon is out on its own with Band 13. AT&T effectively created Band 17 out of spectrum that overlaps, but is not compatible with Band 12, which T-Mobile and US Cellular are deploying. AT&T also has a few other bands that are unique to them or their roaming partners. AT&T and Verizon are also repurposing some of their AWS and PCS band space to LTE. That's why phones and tablets need to support lots of bands today. For nearly 2 decades the carriers agreed on bands but disagreed on the fundamental technology (GSM vs CDMA, etc.). Now they all agree on technology (LTE) but have their own patchwork of frequencies.
