What does it mean when a friend's phone number appears red on iPhone?

What does it mean if my friends phone number is red? I can still text them and call them and they can still text me. But when they try to call me, I don’t get any phone calls?? And this happens to only a few of my contacts! Plus I know these people they are my friends. But when they try to call me their call won’t go through, but they’re able to text me and everything else. Plz help!!

[Re-Titled by Moderator]