Plant-wide pagers

We need beepers or similar devices for our plant maintenance personnel, so they can be paged either manually or automatically by our control systems when their intervention is required. Something like the ones used in restaurants or theaters when your order/table is ready. Anybody knows who offers such devices? By the way, using cell-phones and SMS is not an option. The triggering part should be easy, it is the actual transmitter/receiver combination that I worry about.

Thanks for your help.