I would think the best way, is that you wouldn’t be able to leave a phone message. But some people don’t set up their messages.
But it would be hard to determine if it was by mistake or accident. Like I just recently learned that I have some people on my favorite list but never set their number as actual favorite. I don’t have an I phone though. But my phone allows me to block at certain times of the day, all calls but I can customize it to receive favorites. I will do this at night. I set my phone only to accept calls from close family and friends that I know may need to call me after 10 to 7 am. in the case of an emergency. Funny enough my husbands work number wasn’t on there. And we both didn’t understand why his work cell could not reach me in the morning but his private number could. It wasn’t till I noticed that his work was listed under my favorites but didn’t have a star on it. It had his work cell and his work office and I had to click on a default number that would be his favorite.

Here