You can find some information about VoLTE on Sailfish (or lack of) here: VoLTE support in SailfishOS?. Basically nobody really knows at the moment. The general community consensus seems to be that (a) its not an easy thing to do, and (b) it will only likely happen when it is required by their Russian customer/partner for Aurora OS (the core of which is Sailfish). The rest of the world does not appear to be Jolla’s core market at the present time. Somebody posted that VoLTE was originally in Jolla’s roadmap but has since been removed, others believe there will be increasing pressure on Jolla to provide VoLTE as teleco providers around the world switch off 2G/3G. I don’t think anybody knows for certain what Jolla’s plans are in this area, or if they do they aint saying!