Chamberlain Wireless Pedestrian & Vehicle Alert System (http://amzn.to/19VyRXk) with 2 Way Voice Communication
This is a wireless intercom system that is pretty hard to find where you can use the intercom in 2-way.
We have it on the front and back of the house and it makes different chime sounds for each.
We are thinking about adding two more for a total of four.
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