That's really sad, because it is becoming increasingly common (thanks to the telemarketing industry, robo-calls and scammers) to never answer calls from unknown numbers.This is apparently a pretty common practice now, I’ve seen this mentioned elsewhere. I think the caller either expects you to just call back or might call you back later. Too much time to spend leaving a message. I’ve had people send me a text telling me they had called (but didn’t leave a VM!).
If I get a call and iOS doesn't resolve the caller ID to something in my contacts list, I always reject the call. If they don't leave voice mail, I assume it was a telemarketer or scammer and ignore it.