Simple Answer: It can not be done (I know that).
VoLTE and HD Voice calling are normally enabled on the Verizon branded Galaxy S6 using the MyVerizon app. The MyVerizon app only works for Verizon contract customers with a Verizon Account.
I am using a Verizon branded Galaxy S6 (BYOP) with one of Verizon's many MVNO(s).
Alternate Answer: The only way to enable VoLTE / HD Voice on a Verizon branded Galaxy S6 is if Verizon were to write an app tailored after the MyVerizon app, except for use with Verizon's MVNO customers.
The Galaxy S6 (and all Galaxy's after) support VoLTE and HD Voice calling. Verizon programmers purposely disable the feature for users other than Verizon Contract customers.
Up until now this has been a non-issue. After December 31, 2019 Verizon will turn off their CDMA Voice Channels, making the fully capable VoLTE Galaxy S6 useless when used with a Verizon MVNO .... simply because Verizon MVNO customers have no way of enabling the VoLTE / HD Voice feature.
BTW Verizon: MVNO customer's money is just as green as Verizon Contract customers.
Verizon gets a check from MVNO customers every month (through our MVNO) just like Verizon Contract customers. How about give us some credit & support in this matter?
Thank you for having the best wireless network in the country.