Accepting phone calls from people at work

Question

I got a call today from a friend of mine and i noticed on my caller display that he was calling from work. I assume that he never asked his boss if he could take time off from work to make pleasure calls. I also assume that the boss would not allow him to do such. If these assumptions are correct, what should i do? Should i not answer the phone? Should i answer it and accomplish my mitzvah of rebuke? Rabbi, please tell me the proper thing to do.