Had the same problem, got mine from Virgin Mobile, then it shut down and was put to Optus I think, thats when I think they pushed out the Optus android 8 last update and removed options for voLTE. I have been trying different rom's for the A5 (A520F) and found XSA rom (XSA-A520FXXSGCTL1-20210824105908  -  android 8 Oreo) to have the voLTE and WiFi calling enabled. Haven't tested it yet as I've only just flashed the rom but I can confirm its there and acessible. FYI - XSA is the international version for Aus (Android 6) that Virgin were using when I got the phone in 2017. Hope this helps someone. ... View more