All credit goes to "The Eskimo" on this one...
It's generally true that apps that the user 'quits' using the multitasking UI are not automatically launched again. The system takes that as a hint from the user that it should not relaunch the app. If the user wants the app to be automatically launched in future, they must manually launch it at least once.
For VoIP apps this has been the case since VoIP support was introduced. For other types of automatically launched apps the implementation of this heuristic has been less consistent, although I believe that with iOS 7 it's now applied across the board.