I do something similar with voip.ms (hosted Asterisk).

The intercom calls my voip number, which can be set two ways: 1) play DTMF tone 9 to let the person in, then hang up (which is a security risk if random folks at the intercom buzzed me up trying to get in.

Or 2) plays audio "enter passcode", then:

- if the visitor enters the code that I told them, it plays DTMF 9 to let them in

- if the code is incorrect, plays "incorrect passcode" and hangs up

It also sends an email and SMS whenever someone triggers the intercom so I know about it. With passcodes, I can even set up multiple passcodes to give out to various people (like Amazon, friends) and my notification will display which code was used.

That's pretty cool! If it were me, though, I'd be afraid that I'd forget to turn the socket off after using it.

I guess I'd also set it up so right after the voicemail message runs and lets someone in, it would turn the socket off.

This is cool.

But it wouldn't work for OP, whose door intercom can no longer make phone calls.