Some CMS software allows this via PC. I dont know of a product that uses an iphone app to achieve the same.
You could use a speech dialler and connect the speaker output to a PA amplifier. However the output of such devices is a speaker output not a low level output so you may need a speaker to line level converter. This is not an ideal solution as it is not using the product as designed.
The best way is to use a dvr with 2 way audio, a PA amp, horn speakers etc and a CMS solution.