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It should be easy to understand...
Here are two things you may need to know and then it should be clear.

PBX = Phone System. Think of it like a box at an office that provides all the phones to the offices. Residential phones go through PBXes as well they just typically are what is known as a Multi-Tenant PBX that the phone company uses.

DID = Telephone number :)

ISP = Internet Service Provider

If you know those three things then what I wrote likely will be mostly understandable.

Oh...

SIP is the protocol most often used to transmit phone calls over internet.

Most phone systems and even cell phones use a variant of it.

great! DID is "dialer ID" maybe hehe..nice, thank for the intro to the phone world I can brag about it now lol... thank you sir!! appreciate your time

It technically is Direct Inward Dial. Which is another way of saying Telephone Number. :)

wow ... he he he ...I gave it a shot ;D

just because... :) after I left the comment here I got into this post...my mind is playing tricks on me now lol xP
https://steemit.com/gif/@stellabelle/does-anyone-know-how-to-make-our-profile-pic-into-a-moving-gif