1. In a business telephone system, is the PRI that is used the same as an ISDN PRI?

- PRI is the same as an ISDN PRI, ISDN is the underlying signalling and you can have PRI(23B channels and 1D channel) and BRI (2B channel and 1D channel)

2. If so, is it exactly the same?

- Yes should be the exactly same

3. One a PRI, is there one channel per phone call, so you have a maximum of 24 phone calls per PRI?

Right one call per channel but the max calls you can have is 23. The last channel is used for signalling.

4. How does the telephone system get the phone call outside the building to another caller?

You order the PRI from a telephone company, so on one side the PRI is connected to your PBX and on the other side to the telephone company's switch. This connection means your PBX is esentially connected to Public Switch Telephone Network (PSTN). From here you can call anyone else that is connected to the PSTN.

5. If you have a DID line and want to FAX sensitive documents, is it secure? Should it be encrypted?

I would encrypt the fax if the document is sensitive. They have fax machines that offer security features.

6. What does it mean when someone becomes a POP?

Guess they had a new addition in the family :) Only acronym of POP in telecom I know is Point of presence, is that what you mean?