Someone told this one and I found it online (too long to type it myself):
It's Saturday morning and Bob's just about to set off on a round of golf when he realizes that he forgot to tell his wife that the guy who fixes the washing machine is coming around at noon. So Bob heads back to the clubhouse and phones home.
"Hello?" Says a little girl's voice.
"Hi, honey, it's Daddy," says Bob. "Is mommy near the phone?"
"No, Daddy. She's upstairs in the bedroom with uncle Frank."
After a brief pause, Bob says, "But you haven't got an Uncle Frank, honey!"
"Yes I do, and he's upstairs in the bedroom with Mommy!"
"Okay, then. Here's what I want you do. Put down the phone, run upstairs and knock on the bedroom door and shout in to Mommy and Uncle Frank that my car's just pulled up outside the house."
A few minutes later, the little girl comes back to the phone. "Well, I did what you said, Daddy."
"And what happened?"
"Well, Mommy jumped out of bed with no clothes on and ran around screaming, then she tripped over the rug and went out the front window
and now she's all dead."
"Oh my god... And what about uncle Frank?"
"He jumped out of bed with no clothes on too and he was all scared and he jumped out the back window into the swimming pool, but he must have forgot that last week you took out all the water to clean it, so he hit the bottom of the swimming pool and now he's dead too."
There is a long pause, then Bob says, "Swimming pool? Is this 554-7039?"