You can use a Viking Loop Detect Board at ~$30 or use an ACS Contact Closure Adjunct at ~$135, your choice. Either way, you need to have a device that will do the detection and switching; the ACS on't do it alone.
However..........
There is a contact closure built into the ACS already. The center two conductors in the page port are audio, the two outside those are a pair of contacts that close during a loudspeaker page. If you're handy, you can rig it up and do an intercom 70 and the door will open.
Standard disclaimer: will void your warranty, etc....