Best feature on my phone. Only about one in ten even ring. I try to put businesses that I need to hear from in my directory. Even then I tell them about my ignore unknown numbers feature and tell to leave a message.
Back in the day (15 years ago) I was IT for a newspaper and also managed our PBX. We had 10 POTs lines (Plain Old Telephone - analogue) Out main number was xxx-xxx-3311. A customer dialing that would reach our PBX, which would then switch the call to an open POTs line. An outgoing cll would go out on an open line, never 3311. In the earliest days of caller ID, that was a problem.
One we went IP, we had one number, but could hand many more simultaneous calls than the POTs days. Every all in or out was from our main number. (xxx-xxx-3311)
I still get the occasional business where I've entered their main number in my directory, but miss calls because they come in on another number. Surely they're not still on POTS. My guess is that the main number is a national number, and the outgoing calls come from a local number.
I'd love to hear form somebody smarter than me to explain it.