I get these all the time, but I have my phone set so that the default ringtone is silent, and I have a specific ringtone for every contact in my contact list. Only my contacts ring and these just show up on the screen to be ignored. You can do a little Googling about how you can record your own silent ring tone for iPhones, or you can buy one for 99 cents. The time/money spent is well worth it.
Ironically enough, I decided to do this after the third time my retired dad called me at 6:00am in the morning forgetting I'm in another time zone.