What should I do if I see several ambiguous short messages on my mother's mobile phone and often call a man? My father works in other places.

Maybe your father works in other places and left your mother in the cold. That's why your mother is having an affair with other men. What you have to do is, first, tell your mother some tragic stories about extramarital affairs properly, but don't expose her. Second, let dad go home more, tell his mother that she is in a bad mood recently, and let him call more and care.