Pro tip:
Disable GSM (2G), disable WCDMA (3G), disable VoWiFi, disable VoLTE, disable VoNR.
You still get access to mobile data via 4G and 5G, but you just disabled all technology which allows phone calls. It's now literally impossible.
Drawback: It applies to outgoing phone calls as well. This includes emergency phone calls!!!
A less stupid method:
Check call forwarding settings, copy the number from "when unreachable" to "when busy". If everything goes right, the caller gets "The number you have dialed is temporarily unavailable..." or a similar message when you decline a phone call. For me this breaks with VoLTE enabled, and for the caller it seems as if it just kept ringing. It does however work with GSM-only.
Drawback: This method requires MMI to access call forwarding settings. MMI is only available on 2G and 3G, so your carrier needs to have those. But they may have alternative method for setting call forwarding, or perhaps there's some way MMI (and USSD codes) may work over IMS (the carrier would need to support that). I don't know, I am dumb.
Well... I am using the first method. I usually have a secondary cellphone with me, as a backup, so I could use that for emergency calls ¯\_(ツ)_/¯