VOIP is ok for residential usage, or if you have your own VPN, but I'd avoid VOIP over the public Internet for any commercial usage. At least right now. LD is free, but per-minute LD is so cheap right now anyway, it's not a big deal, especially for small biz. Seriously, for business usage, there is a high probability you'll end up regretting it and upsetting your customers. You'll also need to make sure you have a rock-solid Internet connection with a fair amount of bandwidth (ie, no cable modems). Vonage, all due respect, is scaled for residential usage, and many people are very happy with that. But for biz grade services, you'll get what you pay for. If you really HAVE to have VOIP, check out AT&T which has a biz-grade VOIP service. otherwise call your local exchange carrier and get regular POTS lines. For small biz, most LECs have unlimited LD plans cheap if you ask for it on POTS lines these days anyway. From someone "in the know"...
EDIT: I *do* work for a telecommunications firm. Which is why I think I'm qualified to answer. If you want medical advice, go to a Doctor, yes? And before you ask, my company of employ provides VOIP service, so I'm familiar with what I'm talking about, good and bad. And we have clients that have VOIP from all the major players. Vonage, Sunrocket, Time Warner, and the LECs, and the major IXCs.
He is correct that you can blend both VOIP and POTS lines together, it really depends on what you need the phones for. You can have a couple of land lines, for instance, run DSL over one of them, and the have some extra back up lines (fax lines for instance) as VOIP. My point is, and I stand by it, overall, I have a tough time recommending VOIP as a primary source of connection to the PSTN for small businesses that rely on the telephone for customer communication. That's not to say it doesn't work perfect for some folks, especially if they have good internet connections, a good ISP, and the right CPE to support it. But for most folks home-brewed VOIP solutions have been at best inconsistent. My 2 cents.