"a 2 watt amplifier tied to a small paging horn.
you tie this into the speaker on the phone and that way anything that comes to the speaker is amplified."
I've done this in the past, but I really hate this as a solution. _Every_ sound the phone produces comes out over the loudspeaker, including key presses when programming and going on hold, etc. and dial tone when the user hits a line key.
I'me sure Viking makes something similar that can use a single-line telephone as input (which is what you get from the Aux. jack ot by looping off of the green/red at the block), and give you a loud "ringer" over a page horn (and have external paging through the page port)