Android Intercom App with Sip phone - Part A

I need to build an Android app that will receive a list of residents from and API. when pressed on one, a phone call using FreePBX will be initiated. The resident will be able to speak to the user. If the residents presses #0 on his phone, a string will be sent through socket communication to the door opener, and the door will open. Next phase will be an app that will allow Sip Video Calls.