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.