PBX

You will be very very unlikely to find them because those makers want to protect their intellectual property and guard their profits. If they gave out circuit diagrams then anyone else could have a good try at copying their ideas.

You would needto look on underground or cracker sites. Try looking in alt.* usenet newsgroups related to phonez, warez, and so on.

You could also try second hand technical bookshops and internet auction sites in the book or computer hardware categories.