a good place to start is the book asterisk: the future of telephony. you can download the ebook from www.asteriskdocs.org or buy the dead-tree version from o’rilley (available in many bookstores). it will get you up to speed pretty fast.
Asterisk can, as a general rule, accept any call on any type of line, do anything to it, and if you want, spit it out again on any other type of line.
Now that you are planning to get started, you need to decide what your goal is: do you want to learn *, or just set up something fast.
If you want something fast, try AsteriskNOW or Trixbox. They are self-contained Linux distros which include Asterisk, a web gui, and various other things. However, their web GUI obscures much of the config file from you so it far more difficult to learn *.
If you want to learn Asterisk however, the right way to start is with a bare Linux box and the command line. download the source, compile it, and start learning the config files. The learning curve is steep but once you get a few basic concepts you can do almost anything.
Asterisk can interface with VoIP phone lines and IP phones out of the box using your Linux system’s Ethernet adapter. To use Asteirsk with VoIP phone service, the service must support BYOD (bring your own device) which means that instead of sending you an ATA (analog telephony adapter) that you plug a ‘normal phone’ into, they give you the SIP login info. You plug this info into * and * connects directly over the Internet to the VoIP provider. Good providers- quantumvoice.com, viatalk.com, connect.voicepulse.com.
Asterisk can also interface with POTS (analog) and PRI (t1 type) lines. Full ranges of cards are available from both Digium and Sangoma. When dealing with analog lines you will have FXO and FXS ports. FXO ports connect to a phone line, and let asterisk answer calls and dial out. FXS ports Serve a phone, and give it voltage and dialtone. An FXS port is usually an extension. Both manufacturers have modular cards and go by the system FXS = green module, FXO = red module. A card will have X ports on it, and by installing a module you enable some of those ports as FXO or FXS.
For extensions Asterisk can deal with either analog channels or IP phones (or a few other things). analog channels (FXS ports) are often easier to set up where you don’t want to run Ethernet cable or you want to use an existing telephone. However, on an analog channel you will access advanced features like hold, transfer, conference, etc with hookflashes and star codes. This can be confusing to a user.
I recommend IP phones. IP phones are more expensive but far easier to use- you get physical buttons for things like TRANSFER, HOLD, CONFERENCE, etc. Many IP phones can also do tricks like intercom (voice paging). An IP phone is a telephone that has an Ethernet plug where the phone cord would be, and it will communicate directly with Asterisk over ethernet using the SIP protocol. Most are configured with a web-based system, or you can ‘bulk provision’ them by having them download specially formatted files from the Asterisk server using TFTP. How this happens is different for each manufacturer.
AAstra and SNOM both make good quality IP phones that are popular with * users. Both are responsive to the * community and officially support *. For a lower end / less expensive product, try Grandstream. I suggest staying away from the BT1xx series as they lack intercom and auto-hangup support (when a speakerphone call ends it will play busy tone until you shut it off, annoying+++). Bt1xx however are the cheapest you can get at around $40/unit. BT200 is around $65/unit and GXP2000 (office type phone) is around $100/unit.
Also consider getting phones that support 802.3af power over ethernet (PoE). This allows the phone to recieve its power from a special powered switch, so you don’t need to setup a wall wart for each phone. One good cost-effective choice for this is a Netgear FS726TP (if i remember that right), it’s a 24 port ‘smart switch’, 10/100 with 12 powered ports and two gigabit uplink ports. also has module slots for fiber links. Only around $300 last I checked. Good for setting up a small to medium office with powered ethernet.
Many phones also include a second passthru Ethernet port, so you don’t need two wiring runs to each desk. Plug the phone into the wall and the computer into the phone.
hope that helps!