Talking with Verizon support and reading up on the subject, I don't believe they will have it available for non supported phones. Currently, official Verizon WiFi calling is tied to their Advance Calling feature, which is their version of VoLTE and HD voice. That is only offered to their supported devices. The 6p is capable of VoLTE, but Verizon doesn't offer availability for their home brew version of it to non supported devices. Since there are no plans for them to offer Advance Calling to non supported devices, there will be no official version for the 6p. There's apparently a workaround using Verizon Messages+, but it also requires that you make Messages+ the default messaging app, which I don't want to do either. So the only other option is to use third party WiFi calling apps.
It sucks because I would really like to deep six the two Network Extenders we currently have in our home to be able to make calls there, which isn't possible without them. So much for Verizon's stellar network coverage. Even though my area is shown as having full LTE coverage, it does not. Their excuse was that the coverage maps don't show actual signal coverage, only where they are ABLE to offer service. Seems odd since they call them COVERAGE maps, but after being a customer for so long none of the BS Verizon does surprises me anymore.