How are your 'suggested' friends on the call part of Facebook messenger selected? Is it people you?

Most of them are people with whom you have a mutual friend. Some are just random.

When you get a friend request from a young female in another state, go to images.google.com and drag her pictures into the search box. Quite often the pictures are from another site, or people in other countries, and then you can report that as a bogus profile. A lot of people do that to lure you into a chat room on another site and then they try to blackmail you.