IMHO do not consider vonage or any other VOIP solution unless you have a T1 (or other leased line) connection to the internet. Those are fine for home use over your DSL/cable line, but in a business situation it's just bad practice... if your cable modem goes on the blink or the provider has problems, boom, there go your business phone lines--the lifeblood of most any business.
I use and recommend Nortel stuff. It's pricey but does an excellent job. A 6-line setup isn't too small to consider a key system (basically a mini-PBX). Nortel makes one called Norstar that has been around for years, and scales from a few analog lines all the way up to multiple T1 or PRI trunks. Nortel's phone sets are also pretty good.
The problem is that they can be pretty complicated to install, coupled with the fact that you can't just walk into a store and buy them. You'll need to have a licensed telco installer put them in and do most of the setup/admin. If you're ballsy, buy some used equipment off of ebay (Nortel's retail prices are hugely inflated) and either teach yourself or see if you can pay an installer to install the equipment and support it. It totally depends on how involved you want to get in telecom. It is a completely different beast than Windows/networking and to a large degree still relies on dinosaur-age technology, but it's definitely a good skillset to have.
Look on ebay or call local providers and ask about a Nortel Norstar CICS (Compact Integrated Communications System) plus a Flash voicemail unit. You can wire your fax line into the Norstar and then add an analog terminal adapter for your fax machine/server, or, since you're using all POTS lines you could just wire the fax line directly to the fax machine, bypassing the PBX.
Good luck
l2c
Edit: and if you want to spend a bit more, look into the BCM (Business Communications Manager) by Nortel. It will eventually replace the Norstar system but right now they're still both available. The BCM has the advantage of being Windows-NT based. One thing I hate about Norstar is that normal administration/setup tasks are done through a TUI (i.e. on a phone set using the keypad/soft keys plus a two-line LCD display--not exactly elegant). I basically bought a BCM off of ebay, with a bunch of phones and the appropriate modules (T1, 32 digital stations, 8 analog stations) and taught myself how to install and administer it. Even though all the hard-core telephony concepts/buzzspeak is still there, at least you can administer it with a web-based interface from anywhere on your LAN (or WAN, as the case may be). It's also got voicemail built-in (but requires keycodes to enable) as well as tons of other functionality (including VoIP trunks and IP-based phone sets--but ditto on the keycodes). Nortel updates the software every year or so. A brand-new release, version 3.0 was just put out end of last year that adds several features and bug fixes. Be aware though that if you buy any Nortel equipment off of ebay (or any other non-Nortel licensed dealer) that you have no warranty and no tech support! I have had to rely on kind folks on USENET and mailing lists to help me where I had problems. I saved a TON of money doing it this way (and learned a lot about telecom in the process) but it was a hard road fraught with potential problems. Bottom line is that BCM is a fantastic phone system that will grow well into the future. Being PC-based, it can do things you could never accomplish with Norstar.
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