iPhone only
Not compatible with iOS 6
Allows calls placed from native iOS apps (such as Phone, Mail, and Safari) to be handled by third-party apps if desired
Currently supports Google Voice, Talkatone, Z4, textPlus, Bria, TextNow, and Skype and more
Support for other apps may be added if they can be launched using URL schemes, such as "tktn://call?destination=0123456789" for making Google Voice VoIP calls through Talkatone
Tweak works nicely with appslide
Source code is available here
Configure options from Settings app
See larger Screenshot below..