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Wireless paging system using Arduino. Transmitter / Receiver selection help.

I'm working on creating a Wireless Paging system for a project and I'm having trouble deciding on the transmitter and receiver to use for the project. The system will be used in a shop by person A to request assistance from person B in an office roughly 300-500ft away with a cement barrier and a steel door separating the 2. I just want to do a simple button and display device that basically asks Person B for assistance and notifies Person A that help is on the way on the display. The system must be able to work without interference from the other wireless systems already in use. I appreciate any advice anybody can offer, Thanks!