Your PBX on your Shelf
There are two principle ways to bring Asterisk into your enterprise. The first and most common is called "Customer Premised Equipment" (CPE). That means that your PBX is physically in the same building as the rest of your enterprise IT resources -- that can be a shiny, climate-controlled data center, or in your computer closet.
The biggest advantage usually derived by deploying Asterisk this way is that you can do it on existing retired server hardware. You can handle 100 simultaneous calls on a dual 1Ghz Intel CPU with 1 Gigabyte of RAM memory and 20 Gigabytes of disk space and a single 100 Megabit Intel network card. There is no need to spend several thousand dollars on high-end equipment just to install the software.
It also gives you a level of flexibility with how you connect to the Public Switched Telephone Network PSTN), usually provided by your local Bell or AT&T presence. You can use standard analog copper pairs, digital T1 circuits, Wide Area Network (WAN) Voice over IP (VoIP) links to other branches offices and to an Internet Telephony Service Providers (ITSP) around the globe. You can even blend all of these media to give you least cost routing (LCR) of long distance calls or even completely avoid long distance charges to and from key markets.
Another big plus for this type of setup is the opportunity to have custom-built software running on the Asterisk PBX. This software can work in tandem with your CRM and your Sales systems to automatically route calls based on billing criteria, assigned sales person, place reminder message calls to clients and even allow call-backs from website contact forms.
The biggest disadvantage to having your PBX on your shelf is that you have to handle all the "care and feeding" that goes with having an active, mission-critical server in your enterprise. Highly available servers in a hot-standby configuration, a UPS, telephony connection cards, off-site backups and software updates are things you'll have to consider. If you are VoIP-only, as many businesses now are, you need to consider your Internet link stability and speed; maybe you'll need a second Internet Service Provider (ISP) with an automated fail-over system so that if one Internet link goes down, you instantly switch to the back-up link so you can keep taking calls. So while you may not have to figure a new server into your capital expenditures, there may well be other things that do have to be costed for.
These are all issues that need to be considered and included in your plans to move to a modern Asterisk PBX for your enterprise. For some companies, the capital barrier required may pose too hefty to surmount. However, for many larger companies, the advantages of CPE are too great to ignore.