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.
Semi-offtopic: my life has gotten a little bit better since I used the Android (other OSes likely have similar functionality) option to block any witheld or blocked caller IDs.
Partly because it's rarely anything good these days if it's a witheld number, but mainly because my workplace's outbound calls get automatically blocked so work calls don't get displayed on my phone unless someone uses Skype or a work mobile. Winner.
9/10 would recommend A+++++ unless you're on call, of course.
Fun fact, the store where I bought my GPU, when I had to make a warranty claim, called me from an “unknown number” to warn me the replacement one was in the store (they did warn me they would do it, but it was still weird, otherwise good performance regarding the claim)