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.
Joshjay, I appreciate your feedback and can understand the importance of having VoLTE/ HD Voice work with another MVNO provider that uses Verizon's network. Thank you for your recommendation and I can lift this concern up to our management team. I can't guarantee any changes or support with the current situation considering you're not an actual Verizon Wireless customer.
At the same time, the MVNO carrier may not support all the same bands, network technology nor features supported on Verizon Wireless. Verizon Wireless cannot guarantee full network reliability of service and features when you take your device to a different wireless provider. I hope this information helps clear up the details.
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My MVNO uses at least 3 different cellular networks.
I forgot to mention: Verizon has to be directly involved in the process of writing, loading, and printing the SIMs my Verizon MVNO uses for their customers.
The SIM I am using was attached to a card labeled "Verizon Compatible with a used Verizon LTE device". Stamped on the back is "VW06a" which must be a Verizon Date of Manufacture Identifier. So Verizon knows the SIM Card Number too, obviously.
Verizon also specifies and allocates phone numbers my MVNO uses for their customers which are linked to the SIM Card Number Verizon allocates.
The only bit of information Verizon does not know is the name and address of the MNVO customer using the Verizon managed SIM and cellular phone number.
An account control app similar to the MyVerizon app could easily be designed to allow my MNVO to use their customer's registered Logins and Passwords in place of Verizon's.
I really don't care about details of this. My only concern is will I have to upgrade my Samsung Galaxy S6 sometime this year or not. I would prefer not having to as it is in perfect working condition and is also VoLTE capable.
I really want to wait until several 5G compatible smartphones become available before upgrading. If I can put off upgrading to sometime in 2020, I will be better off with more choices from which to choose.