Im trying to find a way to program a phone to answer my call box and press 9 to open my gate. I saw someone mention that the Asterix phone system with an FXO card could do what I’m wanting to do. Can someone help me understand how this could work.
Asterisk is an interesting application to play with.
1- Connect FXO line in your asterisk box (call box)
2- Call box will answer you call and will ask to dial an extension.
3- on pressing ‘9’, asterisk will execute an AGI that will open a socket and send signal to open the gate.
4- rest your PC interface with gate is electrinics
Keep you dialplan with some security code otherwise anyone can open you gate 
enjoy.
We don’t know enough about your lock hardware to answer properly.
In normal entryphones, the lock release signal is not sent in a way that is compatible with normal phone connections. I’m also not sure to what extent entryphone systems look like telephone exchanges from the point of view of ringing and speech. They generally use special purpose handsets.
Step 3 above is likely to require hardware hacker type hardware to send the lock release signal. It may be direct high current signal to the solenoid in the latch, rather than a logic level signal.