Determine what data network you are using

How do I tell if I'm on 4G or 3G data networks?

Your device will seamlessly switch between 4G and 3G as you move between coverage areas.

4G or LTE or will display on the top of your screen when you're connected to the 4G network.

H will display when you're connected to the 3G network.

4G is a data network so all voice calls are made on 3G.
When answering a call or moving out of 4G coverage your device will use 3G. Before switching back to 4G, you'll remain in 3G until it your device becomes idle (applications stop running in the background). Therefore the switch can take a couple of minutes.