What does it mean when screen says "encryption of ____"?

I may receive calls from 6 different people. Normally, they all say "in call 1, in call 2, in call 3, etc when I receive a call from one of them. This is true for all 6 people. But for two of them it sometimes says "encryption of 1, or encryption of 2". Not always, but sometimes. Why is that? What does it mean? Thanks.