Hi, there. I just developed a similar project with Twilio. Workflow is as following.
Web app has a voice call form and receive screen. Users can call to twilio number or primary number. If the user calls to twilio number, it is forwarded to the primary number, so the receiver can receive the call via browser or his phone.
There is also an inviting interface, so users can invite someone via email or SMS.
Your project is very interesting for me. Contact me and discuss the projects.
Thank you!