In phone call forwarding settings there will be “When unreachable” option with default number. That’s the number which handles “The number you have dialed is temporarily not in service. Please try again later.”
You can take that, copy it into “When busy”, and then when you decline a phone call, it looks like your phone was off.

In my case this doesn’t quite work with VoLTE enabled as it just kept “Ringing” for the caller instead, but hey, at least I don’t have to wait for the ringing to stop.
Just one little caveat: These settings use MMI codes that work only with 2G and 3G. If your carrier doesn’t provide them anymore, it leaves you locked out of those settings unless they provide alternative method to access them (perhaps in their app). Same goes with call barring, CLIR, credit info and all other USSD codes. 4G/5G also doesn’t support traditional voice calls, instead relying on VoLTE/VoWiFi/VoNR or handover to 2G/3G.
This makes 3G my favorite.

Anyway, there’s still a way to reject all calls that way even without such settings when your carrier only has 4G/5G: Disable VoLTE (4G), VoWiFi and VoNR (5G). That leaves you without any way to actually receive phone calls, making you actually unreachable, while still enjoying mobile data and SMS.

The last one can be achieved even if your carrier does have 2G/3G. On Android you can go into one of the hidden settings (*#*#4636#*#*) and in “Phone informationn”, where “n” is SIM card number, there’s “Set preffered network type” where you could instruct your phone to never connect to 2G or 3G.
There’s even options not supported by your device. You’ll have to research that.

Yes, I hate talking to people.