When I first started working with computer I didn't think I would ever do anymore then have computers communicate over phone lines.  But after working in the industry professionally for over 5 years I have worked with several phone systems.  Most notably 3Com NBX and Asterisk.  I have now expanded my arsenal of knowledge to include telecommunications and telephony. 

Presentation on Phone Systems please contact me if you wish to use this.

3Com NBX
3Com NBX is a network phone system that works mainly at layer 2 of the OSI model.  The NBX does have the ability to be encapsulated inside of IP packets so it can travel between routed segments and over VPN connections.  It also has extensive use of multicast (at layers 2 and 3) for control communication. 

I have programmed dial plans and automated attendants on the NBX as well as managed day-to-day user administration.  While I have not written a program as of yet I do know how to program the non-lighted buttons on a business handset to work as Do-No-Disturb (DND).  If you would like details on how to do this feel free to email me.

I am currently working on a program to allow you to sync the time on the phone system from any Microsoft Windows Machine that support .Net framework.  The need for this application came about due to the fact that in spite the NBX being network attached and running POSIX, it does not have the ability to use Network Time Protocol servers.  This is a problem because the system time drifts dramatically over time.

Asterisk
Asterisk is an open source Private Branch eXchange (PBX).  And as with most open software it is available to download for free off the Internet.  Now everyone thank Mark Spencer and the rest of the people at Digium because they are the ones that make this system possible. 

While I have not been using this system long I can tell you it is very powerful.  Due to the fact it is built on Linux is has features that are very unique to this phone system.  The main one being that pretty much anything (and I do mean anything) can be controlled, managed, used and even abused by this system.  For extensions you can use anything from Cisco ATA (FXS) adapters to the sound card in your PC.  As you can see on voip-info.org you have many options for channels with Asterisk. 

Channels?, you ask.  Well unlike a typical phone system where you have defined trunks and extensions, with Asterisk you can use FXS and FXO devices (and devices you wouldn't normally think could be part of a phone system) as extensions and/or trunks.  You can even have roaming extensions that would attach to the closest system and then the systems would exchange information and automatically reconfigure their dial plans for that device. 

Common Channels include: Bluetooth, PSTN/POTS, ISDN (BRI/PRI), T1/E1, SIP, H.323, IAX2, and many more.