HTC uses a chipset in their LTE phones (at least in the Tbolt and Rezound) that allows two things that aren't real well known, and are kind of nice when needed, and are exclusive to said phones:
#1 - it allows for simultaneous voice and data over 3G only (i.e. not on LTE or 4G)
#2 - it allows you to get deep into the modem settings and change the settings from eHRPD (Verizon's technology that's laid over their 3G to help with the 3G/4G handoff) to normal evdo rev a. When LTE goes down, you'll see a lot of posts from people saying their coworkers Droid or iPhone (3G only phones running on evdo rev a) are just fine, but the poster's LTE phone can't even keep 3G. eHRPD is the reason for that. Disabling eHRPD allows the phone to run like a "3G only" phone. It takes a reboot to do it, but if it's one of those days where LTE is down a lot, it can come in handy.
All the LTE phones are supposed to default to 3G when the LTE signal is weak, or when it's nonexistent, like you were saying. However, sometimes they don't handle the handoff real efficiently, and sometimes drop 3G as well. This is why you'll see a lot of signal complaints on the LTE phones - 9 times out of 10 (unofficial stat
) it's not handling the balancing act between 3G/4G real well.
edit: to change that setting, you do ##778# (google ##778# Rezound and it should come up)