Hello, I'm a voip specialist with over 10 years of experience working on the TS-level2/NOC departments of a US voip company; I can install, troubleshoot and customize many SIP devices including: softphones, ATAs, IP-phones and PBX systems. I read your project description and I can help you build the intercom system you want, I'm just not 100% clear how you do the '*9' code, I assume you must have a door gate system that opens the door by pushing a button or dialling a code, is that accessible through your google voice trunk ? Feel free to contact me here through chat at any time so we discuss more on this project.
Thank you.