Student or Learner
1. I would like to know why he(B) wonders what that's all about.
2. I would like to know why he(B) will find out if he(B) listens to his wife on the other end.
A: I found this cell phone on the sink in the men's room.
B: I wonder what that's all about?
A: You'll find out if you listen to his wife on the other end!
2) The person will find out why the cell phone was left in the sink if he listens to the wife on the other end as the wife is yelling at her husband over the phone. If you look at the pictures you can see various symbols (denoting yelling/swearing normally in cartoons) and the receptionist holding it away from his head.
So the cartoon is basically about a husband leaving his phone in the sink as he didn't want to listen to his wife yelling at him.
Hope this helps, jokes don't tend to work if you have to explain them.