A ready-to-install commercial version of this intercom is now available, the Add-A-Com Whole House Intercom for Standard Telephone Systems.
Review and order from the product page at amazon.com:
http://www.amazon.com/Whole-Intercom-Standard-Telephone-Systems/dp/B0037LQ3OQ/ref=sr_1_3?ie=UTF8&s=miscellaneous&qid=1266099630&sr=8-3.
A User Guide is available for download at Whole House Telephone Intercom Update