I'd wait until at least MWC in March to see if a new model or partnership surfaces.

sip hasn't surfaced yet and there is still a lot of stuff that is sort of there but has no UI integration. There is still a lot of stuff that is still broken and not much the community can do about it as Jolla are blocking access because of closed source and unfinished APIs.

If you need SIP, CalDAV, CardDAV today get a mid range android phone and sit it out waiting for Jolla to finish implementing these. I'm guessing it'll be another 12 months at the rate of current development.