There are new building construction codes that require fireproof stairwells where people can go in the event of a fire. The codes also require a battery powered intercom system of some sort be placed in the stairwell and another in a location that a fireman can call into the stairwell to see if anyone is in there.
The WireFree Outdoor Intercom (Set of 2) meets the needs of the fire codes. It is battery powered and can be easily mounted inside the stairwell.