Thanks iJacks.
Android has had native SIP support for a long time (since 5 or 6?). Google's instructions (
https://support.google.com/phoneapp/answer/2811843?hl=en) say it should be there.
Dunno why PlanetC would disable it. I can understand carriers disabling it, but I don't see why PC would.
As for VoLTE, it doesn't show up on mine, most likely because the Cosmo doesn't have the Band IV that my carrier requires.
My Carrier (Freedom Mobile in Canada) is supposed to have VoLTE support, but I kind of screwed up when I checked the band compatibility for the Cosmo on my network. Freedom requires Band IV (1700/2100 AWS), so whilst I have text and data working, calls do not.
I was aware this might be a problem, and I would rather keep my Cosmo and change to another network than stay with Freedom.
In the meantime, though, I was hoping I could get VoLTE working or SIP at least, until I can switch.
(I do have SIP working via MizuDroid, but I was hoping to use the native dialer)