PostmarketOS has VoLTE support for the Pinephone. Maybe it’ll be upstreamd to ofono and get’s to SailfishOS that way.
https://wiki.postmarketos.org/wiki/PINE64_PinePhone_(pine64-pinephone)#VoLTE
Dual Transfer Mode
Dual Transfer Mode (DTM) is a protocol based on the GSM standard that makes simultaneous transfer of Circuit switched (CS) voice and Packet switched (PS) data over the same radio channel (ARFCN) simpler. Without DTM, the mobile device must be capable of reception and transmission simultaneously (full-duplex) requiring complex and expensive circuitry in the mobile terminal. With DTM this requirement doesn't exist and makes the device implementation simpler and cheaper. DTM is a 3GPP feature intro...