Could you check the following on your CMF phone, to validate if the phone is actually working over VoLTE (i.e. registered for IMS)?
“Call” *#*#4636#*#* - this will bring up a menu screen. In the upper right corner, a “…” dropdown menu can also be accessed; in there, choose → IMS status. If IMS status is “registered” (or similar), you’re actively using VoLTE. Otherwise, the phone has dropped off into a lower protocol, and not actually using VoLTE.