Certainly possible. Any door entry system that supports DTMF should work for you. You may require an ATA to pass DTMF to the door entry device. We've used devices from these guys in the past www.vikingelectronics.com
When the user presses the intercom button it rings the ip phone so you can have a conversation. The device can be setup with a code so that when security have finished talking they type in the code on the ip phone and this unlocks the door.
We have several of these. We use a remote door box, which hits a ring generator (Valcom), the ring generator then comes into an FXO port on the router, that FXO port rings whatever number we need to ring. The user then answers on the IP Phone and dials whatever code is needed to open the door for the person on the other side. I'm not sure if these is the scenario that you're looking for, but this is something that we've set up a few times in the past.
Quite simple, register a Cisco Certifed Intercom to call manager. Set a DTMF (Dual Tone Multi Frequency) code. For example 321. I bought a Cisco approved Intercom recently. You can dial into the Cisco IP Phone Network and any Cisco Phone can control the door. For this application we was triggering an automated gate. Bought it from here
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