1. Open dialer
2. Enter *#*#4636#*#* (*#*#INFO#*#*)
3. Select “LTE only”, “NR only” or “NR/LTE” as applicable or preferred.
4. In settings, navigate to mobile network
5. Disable VoWiFi, VoLTE and VoNR

No more phone calls!
If there is no 2G/3G in your area, you can just skip to step 4.
Outgoing calls won’t work either.
USSD/MMI including settings like call barring and forwarding may also not work (set/reset, existing settings should be fine).