Couple of things I've found in my research about this issue as it applies to Sprint:
This has nothing to do with the device, this is a Sprint tower issue related to the Spark Network
The device is working as intended...it's the tower that's not...shocking
The Spark modem implementation is a result of the problems with the towers not the cause
After the local tower is updated with LTE, it also must be updated with additional software related to the hand off of 3g activity (necessary for calls and texts on Sprint) back to pure data.
This has to do with the way that your device interperts "Spark" services as it relates to the tower - the issue is that "Spark" requires three points of connection to be "Spark" if you don't live in a "Spark" network, the tower is supposed to identify and then let you connect back as "Normal LTE" (if that makes any sense)[/INDENT]
This will affect any Sprint device with a Spark modem (going back even to the Nexus 5, which I own...and did experience the problem as well)
[*]Had (and still have in some areas local) the same issue - but the minute the towers were updated nearest me, the problem is eliminated.
For instance, at work - no issues, on my drive home - if I get a call, and I'm past tower A (updated) and attached to tower B (not updated), the 3g network picks up the call - and will not connect back to LTE until a forced modem reset (or Airplane mode toggle).
Last point - yeah...it sucks...
Jack