Phone up someone in your business that is in a different office (or out of hearsay). Ask for Mr Walls, when you get the response no one called Mr Walls work here ask for Mrs Walls. When they reply that Mrs Walls doesn’t work there ask if they have any Walls. Once the person on the other end of the phone says we have no Walls in the office, quickly scream with a panicked voice, “quick get out the roof will collapse!”

Putting on a different accent and making sure no one called Walls works at your business helps the joke to work!