Who is calling me?

There was a lady living in a large house that had two floors.  One day, she was sweeping the ground when the phone rang. RING RING.  She picked it up and said

"Hello?"

"I am going to be there in 30 minutes.  MWA HAHAHA!"

Then the scary sounding man hung up.  'It is probably just a prank caller' thought the lady.  But, 15 minutes later the phone rang AGAIN. RING RING.

"Hello?"

"I am going to be there in 15 minutes. MWA HAHAHA!"

The lady began to get frightened so she called the phone operator to see where it was coming from.  

"Hello?" said the phone operator

"Ummm... Hi.  Someone has been calling me repeatedly, and I want to know who it is."

"Ok.  Can you tell me your name, where you live, and who lives with you?"

The lady told her the information.

"All right.  Next time the person calls, call me and I will see where he is calling from."

10 minutes later the man called again.

RING RING.

"Hello?"

"I am going to be there in 5 minutes. MWA HAHAHA!"

The lady then called the phone operator.

"HE CALLED ME!!!"

"Ok ma'am. Calm down.  We will figure it out.  Hold on... I am tracking...  *GASP*  the phone call is coming from... UPSTAIRS!  Get out of the house immediately! I am calling the police!"

A few minutes later there was a thump from upstairs.  The lady freaked out and ran outside.  The police went in the house.  A couple seconds later they came back out with the man from upstairs who was carrying a knife in his hand.