SIP Qoutient -- Best Explained -- Just that if this understanding was there when I started 10 years ago

From SIP sense .com

Why SIP?

https://www.youtube.com/watch?v=BNESo_SwtNA

SIP Devices In A SIP Network

https://www.youtube.com/watch?v=uTU6-gDQL_E

What is SIP?

https://www.youtube.com/watch?v=erICfPV8-Lg 

As they say, when you lost/confused/fired  start from scratch, and these 3 videos does the same.