Voice over IP (VoIP) is the new digital phone technology that is revolutionizing the telecommunications industry. By digitizing voice communications companies can compress the data and send a lot more data over the same connections. They can also reduce signal degredation over long distances since the signal can be verified along the way and reproduced exactly. By linking the calls to computers more directly it is also possible to achieve a much higher level of systems integration than you could with analog phone systems. Add to that one of the major players in the field being an open source software solution and you get virtually limitless possibilities.
In our case we have completely eliminated analog phones except one emergency phone for backup purposes. All of our phones are interconnected including our cell-phones. We have a single voice-messaging system that will email us our messages or call us with them if we are out.
One of the key factors in being able to setup and experiment with Voice over IP is the availability of a free server platform. In the traditional PBX model you would have a proprietary box sitting in your office somewhere which would connect to (you guessed it) proprietary phones which only work with that one PBX box. Ah the joys of vendor lock in. Well with the advent of VoIP and the SIP protocol we now have an open playing field. Add Asterisk, an open source VoIP PBX implementation, and you also have an open server platform to build off of. So what does all this mean?
Well... First off now anyone with a spare PC can setup a VoIP PBX and connect up a few soft phones and have a phone system. You can then go and add any number of hardware phones from any number of vendors (no vendor lock in here) and place calls between them. If you shell out a few bucks you can even tie that new phone system into the rest of the world. This can be done either over the interent to a VoIP provider which will sell you a phone number (pretty much anywhere in the world) or by adding a special card to your server and plugging it directly into the public phone system. Special devices even exist to attach your phone system directly to the cell network. Basically now anything can talk to anything else all using a common language (SIP). Not bad eh?
Hardware Line Cards
Your best and most reliable bet if you want to connect your server directly to the public phone network (PSTN) is to purchase a hardware line card. While there are a number of vendors who supply such cards we would recommend sticking with those supplied by Digium
if you are using Asterisk. We did try some of the cards (both analog and digital) made by Sangoma and had nothing but trouble from them. The drivers were buggy and constantly out of date. If you called them you could get beta driver but even they didn't work reliably. Crashes, lines vanishing, problems even getting the things to compile and install. In the end we said enough is enough and decided to stick with Digium. Their stuff just works. No muss no fuss.
In our home PBX we use a TDM400P PCI based card to connect up our analog phone line as a backup for our internet based primary line. This card provides up to four ports for either internal analog phones or external phone lines. You can configure the card however you want by purchasing the appropriate modules for each port.