Yeah yeah, all nice and dandy, but... really you want all this beautiful communication platform to only be useful for calls between internal registered users?
Let's see how to add a SIP gateway to an Internet Telephony Service Provider (ITSP), the easiest way to expand our communication platform to have a global reach.
An ITSP provides its customers with SIP "trunks", eg: capabilities to connect outbound calls to the world phone network. Also, an ITSP can provide DIDs (Direct Inward Dialing) numbers: telephone numbers that when called originate an inbound SIP call to our communication platform. So: SIP trunks are for us to send out calls to the world. DIDs are for gathering calls from the world to us.