There has been some discussion the in the forums about this in recent days, see the recent posts beginning here:
I know that /e/ supports the feature: it’s working on some devices (e.g Xperia XZ1 lilac) but not on others (e.g. Xperia Z3 Compact z3c, Z5 Compact suzuran). If it is available on your device, you can find the button to switch it on either in Settings > Network and internet > Mobile Network > Advanced, or by seraching in settings for ‘Calling’.
If that switch is present but inactive (i.e. greyed out, can’t be pressed), that is probably (I say ‘probably’ as I don’t have a SIM for a provider that does not support it) because your network provider does not support the feature.
I hope that helps. Good luck