I wrote about VoxOx earlier this year when I saw the Mac app at CES, and the company has now released an iPhone version that replicates the company's call back service -- they'll connect two different voice lines to each other over your phone. The app can even accept SMS messages, pull off a 20-way conference call, and even take in faxes. There's a video demo of the app online, and it's a free download, though you'll need a VoxOx account to call out.
A few of the reviews on the app say there might be an issue with some incoming calls, but given that it's a brand new release, there may be a few hiccups that need to be fixed in future releases. That said, if you're a user of the VoxOx service, this app is a no-brainer, and even if you're not, but still looking for an all-in-one communication solution, it might be worth a look.