Almost all large department stores and mall shops have employee and PA phones all over the store. Try to identify the main PA system line button, then locate an isolated station where you can use that phone unobserved.
Compose the most objectionable statement you can imagine that you can deliver in about five seconds. Make it gross, sick, insulting, or obscene (best if you can combine all of them). Write it down. Then read it over the store’s PA line. Hang up the phone quickly and walk briskly away. Look as shocked as the rest of the customers in the store who heard your message. Do it again another day or at random intervals. Always end by saying “This message brought to you by the management of (*store name*).”