Looking for help in finding an intercom type system for an office/ warehouse.?

My company is interested in purchasing an intercom type system that would allow the secretary in the main office, to notify employees in the warehouse of telephone calls and stuff of that sort. It would also need to be really loud to make it over all the noise of the shop. Does anyone know of a good system or even what that type of system is called? I've tried to look it up on the web, but I must be using in the wrong name for it because I've had NO luck. Thanks so much for your help!