*#*#INFO#*#* (~equal to *#*#4636#*#*) gives you status information. Imo most useful to show if IMS registration is successful. It too will tell if vowifi (or volte) is available at that point in time. When I checked on my FP3, it said “wificall unavailable”, rebooted to trigger SIM re-registration, wificall was available and it made the next call on wificall.
Do you do anything custom for DNS? providers want you to use their DNS for their tunnel endpoints to work.
tunnel?..
VoWifi is a negotiated ipsec tunnel on UDP port 500 / 4500 with your providers endpoint - then “volte” minus the -lte happening over that. One can passively watch at the router level if it occurs.