This is for those that are really into building their own hardware, in addition to being familiar with Asterisk. From Reddit’s Asterisk section comes this post:

I’m working on using a Raspberry Pi with Asterisk installed on it to act as a door entry system integrated with my home phone system (also running Asterisk). The operation I would like is this:

    Doorbell pushed.
    Raspberry Pi detects push and its asterisk originates a call from the local console channel to some predefined extension on the main asterisk server, which routes to the appropriate house extensions.
    Person at door hears either ringing or music, whatever.
    Call is eventually answered.
    Caller talks to owner and owner decides to let caller in.
    ????
    Asterisk in Raspberry Pi activates a local GPIO pin to open the door strike.

I believe I’ve got all of the above described parts working, or at least workable. The “????” line — I haven’t.

The post continues to explain the thinking that has bee put into solving this problem. Then others chime in, and finally a solution is found that involves some lines added to Asterisk’s features.conf file, and a small program in C. We’re not into hardware hacking but this solution seems like it would be a lot less expensive, and probably more fun to build than some, than an equivalent commercial unit.