Communicate with the World via SIP Gateways and DIDs

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.