1st try putting the phone in airplane mode for a few minutes then turn airplane mode off
if that doesn't work, reboot phone by holding home and sleep-wake button together for 10 seconds.
IF that doesn't work, reset network settings in settings-general (you will need to redo wifi and Bluetooth connections after this )
"Should be using 4G" is an over statement. 4G LTE is not a promise, it's more of a bonus. What phone do you have? Was it using 4G LTE at the same location prior? Have you installed any apps or updates? Are other Verizon customers getting 4G LTE in the same spot at the same time you are getting 3G?
1x is 2G
IT is expected to drop to 1x when making or receiving a voice call, as that is the way voice is Carried on all Verizon phones, except the very latest (like iPhone 6) that has advanced voice over 4G LTE
you know that BIG RED map of the united states that Verizon advertises to show their network coverage? Its not really that accurate, where I live and the places I have gone. The 4G-LTE network can be spotty, and drops in and out pretty much at random. I found this to be the case with either my 5C or Sony xperia. Theres not much you can do from an iOS standpoint to improve this... its more a network coverage and strength thing. Furthermore there are times and places where the 4G network is really slow too, not much faster than 3G.
One thing you can do is download a network speed test app like ookla, and use that to benchmark locations around you. So for example I already know www searching for price matches in my local Frys electronics store is going to be very slow.
I'll tell you one thing you can do (definitely can't hurt), is vote in favor of cell tower construction measures when they come up in your local elections. My area in Palo alto has very weak 4G-LTE coverage, fortunately the 3G coverage is excellent and I almost never see the dreaded 1x... which is really slow.