drupe allows you to select your default app to complete some of its various actions: call, SMS texts and email.
To do this, go to drupe settings > General > Applications. To the right of the apps, in light blue, there'll be written the name of the app this action icon will start, or it'll simply say, Set default. Tap there, and the list of available apps will be displayed. There, you can choose which app will be your default.
The only difference is with the call action icon. Since the call action has to go through the device's infrastructure to make a call using the cellular network, the first time you make a call via drupe, you'll have to select one of several available apps to complete the task, then select whether to save this selection by tapping Always or choose to have the selection pop-up be prompted again in the future.
If you've selected an app once to always complete the task, and would like to change it, you'll need to do it this way: device settings > Apps > App manager > All > find the app that has been selected before, tap it > Tap "Clear defaults". Once that is done, you'll get the selection pop-up again.