VoIP provider Toktumi has a solution that may work out well for you. They're now selling an app called Line2 (US$0.99) that mimics many of the capabilities of the standard Phone app, complete with a form of visual voice mail. Purchasing the app provides you with one month of free VoIP service. After that point, each month's continued service costs $14.95 for unlimited calls within the US and Canada. The service is purchased in-app, and was a point of confusion for me -- the app listing in the iTunes store shows a "Top In App Purchase" at a cost of $19.99 for a month. Toktumi's website and the promotional video (above) do show the lower monthly rate.
While many VoIP solutions require a Wi-Fi connection to work, Line2 works over both 3G and Wi-Fi. The app has many of the features you might want in a business phone, including call waiting and call transfer, the ability to create conference calls with up to 20 participants, and even integration with Toktumi's hosted PBX service.
For small businesses that want to present a more professional image to callers, Line2 might be just the right solution. Toktumi notes that the app also works with both the iPod touch (Wi-Fi only) and iPad (Wi-Fi or 3G).