… since before Jolla started in 2013: By Nokia.
And an almost working sofia-sip is part of SailfishOS for long (since before 2015): https://together.jolla.com/question/415/sip-voip-native-integration/
But the last time Jolla did work a tiny bit on this was in 2015 per SailfishOS 2.0.1, while their roadmap once stated VoLTE will be ready in April - May 2015. One may still install and configure it to the point where it mostly works (although without GUI), except for receiving calls (for both, see aforementioned link).
BTW, there once was a “Jolla’s biggest customer”, who direly wanted this feature (VoLTE): Intex, the Indian company which sold the Aquafish phone (basically a rebranded Jolla C with a slightly different modem). Because JIO, India’s low cost mobile network provider started with 4G (“LTE”) and never offered voice services, only VoLTE (which uses the data channel). Many, many Indian SailfishOS users were extremely frustrated in 2014 - 2016 (then they vanished), because they were never able make calls with their Intex Aquafish phone and a JIO SIM card.