Searching for a reason why I can't send emoji laden texts through Textra on my Nexus 5, I came across this article, http://s4gru.com/index.php?/blog/1/entry-357-nexus-5-and-lg-g2-experience-temporary-sprint-lte-connectivity-issues-due-to-circuit-switched-fallback-technology/, that sheds some light on how Sprints tri-band network switching works. For those of you that don't want to read the whole article, here's the important piece...
_"...In previous Sprint LTE phones, when a device was in Sprint LTE coverage it would park in both the LTE and CDMA Sprint networks at the same time. When a voice call came in, it would just go straight through to the device. And signal to the LTE network would be maintained the whole time while the call was active.
In contrast, a Sprint Triband LTE device can only stay on one technology at a time. CDMA or LTE, not both. So when a Sprint LTE Triband device is in Sprint LTE coverage it parks only in LTE. And doing so means it cannot transmit calls without Circuit Switched Fallback (CSFB) on the network side. CSFB and eCSFB (Enhanced Circuit Switched Fallback) are network controls that will allow a single mode/single path network to operate in two modes, both CDMA and LTE.
Here is how it works in the simplest way I can describe. When your Triband LTE device has an LTE signal, it cannot receive or make calls on its own. It is just using LTE data happily. However, what if someone calls you? How does it get through the CDMA network to your device? Via CSFB.
When the Sprint network tries to forward a call to your device but cannot see it via CDMA, it then checks for an LTE connection to your device. If it sees one, it tells your device to disconnect from LTE for a moment and reconnect to CDMA. Your device then jumps over to take the call on Sprint CDMA and the LTE session is interrupted. This happens very fast and seamlessly. Except for the loss of data availability. If you receive a text, the Sprint network is able to route it to your device via LTE.
Circuit Switched Fallback is a great solution to the issue of Sprint Triband LTE smartphones. But the problem here is that the Sprint network is being upgraded in Network Vision, and not all Sprint parts of the Sprint network can currently support CSFB. And it affects all Sprint Triband LTE phones, not just the Nexus 5 and LG G2..."_