Recently a friend of mine asked me to recommend a business telephone system for his (starting out) small business. He needs to be able to connect someone somewhere else as though they are in the same office. This is becoming more and more of a basic need for all businesses- the modern office is really made up of several people's home offices, scattered geographically and connected via broadband, with no real "center". Nonetheless, I took a moment to highlight my experiences regarding traditional PBX systems as well.
We have a Samsung Officeserv iDCS 500. I personally don't find it to be easy to manage at all. I have received many requests that would be very straightforward to resolve on other systems, which have fallen dead due to lack of clear documentation and poorly implemented management software. At $6,000, I don't even think it was that cheap. To its credit, however, it does make the process of provisioning off-site VOIP extensions a fairly no-nonsense operation, as we have found.
If you are going to purchase a phone system for your office, and you
expect it to scale, I would stick with the tried and true "gold standards" of phone systems. Buy something made by Avaya (partner) or AT&T (merlin). There's tons of support, hardware vendor choices, and documentation available. Seek a second hand phone system on ebay or websites like usedphones.com that have put together nice packages. Make sure that you choose one that allows VOIP extensions and voicemail. Be prepared to spend around $2000 to start, and $100-$200 per phone. In my experience, this is a better alternative to buying a cheap phone system that has idiosyncrasies, and that you are stuck with for a long time.
If you are really strapped and aren't afraid to get your hands dirty, you can put together an Asterisk box. You would need an older PC that has several full size PCI slots available (free-$300), a PCI FXO card (for your telephone line) ($200-$300), and several inexpensive IP phones, such as the Grandstream Budgetone ($100/ea). Again, this system is not for the faint of heart, and there are known difficulties with getting these systems to work from behind a firewall. A commercial approach that is similar to this, but doesn't have the headaches of setup is http://www.fonality.com. Their entry-level server product is $995.
Ok, the last one. If you are not set on having a traditional dedicated, owned piece of hardware, you could save a lot of the cost of entry by trying out a hosted PBX service, such as Ringcentral or
pbxes.org. I have no experience with this, but it may be right for a small company, with no "central office" per se. One disadvantage I can think of is that you are depended entirely on your ISP, and someone else's phone system, to maintain your business' communications. Should they have a company-wide failure, or "spontaneously" go out of business and pull the plug one day (like SunRocket recently did to their residential customers), you are down for the count. Nonetheless, they claim to have service starting at $9.95. If you check this out, please let me know how goes.