Does it still apply if I set the "If busy" forwarding to number that handles out of service call attempts ("The number you have dialed is temporarily not in service. Please try again later.")? They could see it was ringing shortly before being redirected, instead of getting redirected immediately. But it's unlikely someone thought of that. And yes, I still get SMS that someone called me when I "was unavailable".

Unfortunately, this doesn't quite work with VoLTE enabled. It just keeps "ringing" after the redirect instead of playing that message. I disabled VoLTE on my phone for that reason (falling back to GSM). Although that's still better than nothing.