The story is: Yet now that Verizon has officially begun to roll out their VoLTE service (called “Advanced Calling 1.0″), customers are only given two choices that are capable of using the VoLTE service: the Samsung Galaxy S5 and LG G2. See Verizon rolls out VoLTE to just 2 phones - Android Authority
It is pretty crappy of Verizon to not allow the LG flagship phone the capability.