You can use VoIP SIP SDK to make a custom intercom.
Obviously you can make a connection to communicate before you open the door.
However, one of the basic features of this solution is to be able to see the person near door before actually making a connection.
This feature is typically called early media.
How to implement it for Android:
when received incoming call in Android app invoke method:
phone.answerCall(183, true);
This method sends to remote side SIP request “183 Session progress+SDP”,
which allows start sending audio\video in both directions, but call still not answered.
which allows start sending audio\video in both directions, but call still not answered.
Then you can answer or reject call.
If you have more questions please contact us.