Why RS485? Whats wrong with a simple 3-wire connection (monospeaker, monomic, ground) where you short monomic to ground on button press?<br>
Then you could use a simple usb device + device server to convert fron "smartphone headset" to usb then to network.<br>
On the server, you use a SIP phone client, who use this device as mic/speaker, which is configured to lift the hook on headset button press.<br>
In asterisk dialplan, you have logic which automatically dials where the doorphone should call upon hooklift.