I. Deployment  / Architecture Block Diagram

II. Resources Used

sip proxy server + sip user agent

1. flexisip / freeswitch + linphone / pjsip / libexosip2 + libosip2 + ortp + mediastream2

III. Each module concrete realization

Maybe you don't need to write a single line of code, because all modules are perfect.

1. flexisip

https://github.com/BelleDonneCommunications/flexisip

2. linphone

http://www.linphone.org/

http://www.linphone.org/sites/default/files/solutions-intercomsystems.pdf

http://www.linphone.org/sites/default/files/solutions-securedcommunications.pdf

3. freeswitch

https://freeswitch.com/

http://www.freeswitch.org.cn/

4. pjsip

https://www.pjsip.org/