Level of Difficulty: Intermediate

Armed robbers, Chen, Baekhyun and Luhan, invaded a bank. They were busy looting when suddenly a phone rang. The phone happened to be at the reception. Bekhyun asked the receptionist to attend the call and talk without giving away the situation. The call happen to be from her mother. 

She spoke, 'Do you have any emergency mom? Can you give me a call when I get home, I could really use your help in buying new curtains?' Then she hangs up.
The robbers are busy when the police arrives suddenly along with the mother of the receptionist.

How did she know about the robbery?

Solved by KaiSooEXO,she's officially a GENIUS !

Answer: The receptionist was pretty clever she played with the mute button of the phone while talking with her mother. She muted everything except the word emergency, call and help.

So while talking, she sounded like 'Emergency… Call… Help' to her mother and thus she called the police.