There is an open source app called openvbx, for the iPhone that has not been updated to work with iOS 7. I would like for you to take a look at the code and see if this is something you can update to work on iOS 5 and above.
Known Bugs
Crashes
Crashes when sending text message
App crashes after it is closed and reopened for the first time
Use basic design elements for UI i can provide app icon
Thanks,
Mark