Why RS485? Whats wrong with a simple 3-wire connection (monospeaker, monomic, ground) where you short monomic to ground on button press?
Then you could use a simple usb device + device server to convert fron "smartphone headset" to usb then to network. 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. In asterisk dialplan, you have logic which automatically dials where the doorphone should call upon hooklift.
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