In Chinese, the word for mistress is 小三. It's a word you'll encounter quite often in magazines and TV shows, so it might be worth remembering.
In today's elementary example, a lady unexpectedly answers the phone when Little Ming calls his dad. Could the father possibly be having an affair?
“Mom, dad didn't answer. It was a lady who answered.” Who would've thought that dad has a mistress!
In our second sentence we encounter a 是... 的 construction. This is used to emphasize that it was not the dad who answered, but a woman.
You can read more about this grammar structure on the Chinese Grammar Wiki.