No matter what type ringer unit you use, as long as the phones don't necessarily have to all ring at the same time, you can build a simple switch box where at the push of a button you can shift the ring voltage to any number of phones.  Bottom line is that on the "in" side of the box is your ring voltage. The out side of your box run to the individual phones. (there is no real limit to the number of phones) for each phone there is a normally open button associated. when that phones button is depressed, the ring current goes to that phone, when released it stops. then push the next one etc etc etc....