As of technology today, you actually have to be in the store to page or get on the store's intercom. Since the Fred Meyer's Prank of 1994 technology has changed. Back then the intercom was an actual extension that could be reached like any other extension; but with advancements in telephone systems, the intercom is now (in most stores) an add on to the system, kinda like on-hold music. It has to be reached by pressing a button on an actual phone that is hooked up to the system to page over the intercom.
Hope it helped!
RE:I will stand corrected as I have seen examples where one would be able to get on the intercom in Home Depot stores, but not live. This is an example where the computer would have to record, then play it back through the aux jack.