Right now with LTE, it's only data. There is no voice on LTE, on the carrier's spectrum. Yes, you can use Google Voice on LTE, accessing the internet, but that's a workaround.
With LTE phones -- ALL LTE phones, even Verizon -- for voice you use the carrier's other technology. In the case of Verizon and MetroPCS, that would be CDMA. So, you are talking on CDMA and browsing/downloading email/facebooking on LTE. Thus, you are using two radios at the same time, and battery usage sucks.
Or when you were on the phone with your cousin, might have even been on wi-fi rather than LTE.
It's just with CDMA-only phones like the Optimus, you can't talk and browse the same time while using CDMA. If you are on a wi-fi network, with a phone like the Optimus you can talk on CDMA and be on the internet at the same time.
But with VoLTE, you will be able to talk and browse on LTE at the same time -- thus using only one radio and your battery life will be so much better.