Keep calling my cell # and not leaving any message either.

Called, but didn't leave message.
I called the # back and its for a "Rebate Reward Redemption"

"I'm sorry, that's not a valid extension, please try again" *beep* *beep* *beep*

Wait, you called *me*. I don't have extensions.

Is somebody trying to establish a 3-way call or something?

Called just today and didn't leave a message.

Called my cell phone and didn't leave a message.