I use
Kall8. It's wonderful for someone in my situation. It only costs me $2 a month to keep the number open, plus 6 or 7 cents a minute to use. That may sound steep, but in a typical month I have NO calls since my clients are all mainly email-based. I just keep the line for potentials and to do quarterly follow-up calls with current clients.
That said, it would definitely work for a business that has bigger phone needs. There are lots of options, nice packages that include multiple outbound messages, menus, and separate extensions if you have more than one person in the business.
But like I said, it's the basics that sold me:
--You can have the same number for phone and fax (for fax, it's sent to me by email as a PDF @ 7 cents per page), so I had no expense in buying a fax machine.
--You can set your toll-free number to forward to any other number, land line or cell, so I have it forward to my home line. That means I also don't pay extra for a second line.
--You can set the ring forwarding to a schedule. My business line never, ever rings before or after hours or on weekends, so I don't have to worry about screening calls during "off" times. It goes directly to VM instead.
--You can make outbound calls from the number, too, so my clients never see my private home number on their caller ID even though that's where I'm calling from.