When we develop a mobile app, is it important to add interruption testing as another test strategy to ensure that the app behaves correctly under different interruption conditions ( incoming calls, SMS etc.). If yes, how often it should be incorporated into app's release cycle?
Just like most things, the answer is a bit of an "it depends" situation. Generally speaking the more your app functionality/features/experience is tied to calls and SMS then the more probabilistic it is you should need to test the interruptions. FWIW my firm has helped launch dozens of mobile apps and I cant think of a single instance where we tested this with any regularity. Generally speaking though if you were to want to test it regularly then I would just add it to the list of regression scenarios your team tests after making updates to the application and makes sure nothing changes/alters unintentionally.
Personally I have never seen anyone do this nor do I think it is particularly necessary. You do want to make sure you check what your app does in a paused state (incoming call, app switching, rotation) if you need to save data or are showing sensitive user data.