I use Vonage for this business, and MagicJack for another biz I do part time gigs for. The Vonage is nice because I set it to ring my office and my cell at the same time, which ever one I pick up gets it, or if I don't get it, Voice Mail picks up and emails me the message, but it's like $25/month.
The MagicJack is good just to "keep the number" and forward to another number for $20/year. I've used it and had decent results too. They just now came out with one that plugs directly into your router, so you don't have to have the computer on. I ordered the hardware but haven't hooked it up yet. My wife uses it at home and it works well for her too. You kinda get what you pay for, but for $20/year it's an easy way to have a number forwarded to some other number if you need, or to even just use for Voice Mail to be sent to your emails.