I'm trying to understand how to properly configure a PBX (Private Branch Exchange). I've stumbled into a few brick walls, and I feel like the answers to these should be obvious, but the Internet has not helped at all. After consulting stackexchange.com/sites, this seemed like the best place to post regarding telephony networking,
First of all, does a PBX require its own power supply? Standalone telephones work in a power outage since they are directly connected to the central office, but in a PBX setup, the telephones are connected to the PBX, which is linked via outside lines to the central office. So, would the PBX/telephones work in an outage, or does it need backup power? If your PBX is Centrex (hosted at the exchange), will the telephone company supply power to your PBX in an outage?
How does one actually configure extensions in a PBX? How would one set the system up so if you dial '1010' it rings a specific line somewhere in the building? Since there is no software or computer-interfaces in a PBX, and it is analog, rather than digital, how does the system know which line is which?
Also, does each outside line need to be paid for individually, or are there trunk leases? If everyone dialed out at once, would some users get "no circuits are available" if you have 50 internal lines but only 25 outside lines? Would you need as many outside lines as inside lines? And I know many organizations have assigned prefixes (they own all the numbers in 555-345-XXXX) where they own all the X numbers. Do you just ask the telephone company to lease an entire prefix, or part of a prefix?
Also, if the PBX loses power, can I just unplug an outside line from the PBX and plug a telephone into it? Is there any reason that wouldn't work? (like if the line is part of a trunk line instead of an individual cable)