I use a system by Optex. 4 rf zone transmitters, each with a distinctive ring. The 4th has a dry set of contacts that you can use to trigger other things (like maybe turn on the electric fence).
Optex Wireless 1000 Series Annunciator System : TD-10U , RC-10U , TS-10U, RG-10U , and RCTD-10U Kit
FYI on zone 4; It is a latching circuit - once activated it will remain in that state until manually reset by human intervention. So if you hook it up to an air raid siren and go on vacation, expect some ill will from the community, LOL.