Powered Phone Amplifier with Horn works really well. I would certainly be able to hear it up to hundreds of meters away on my weekender property. You'll need speaker cable, soldering gear, cable shrink and maybe some grommets to connect and run the speaker outside, and (optional) a standard phone wall mount if you want to mount the control unit. I mucked around with an unpowered unit from Jaycar that was just rubbish, this is the ants pants!
