I have a design of activities like this

I have one main activity and 5 subactivities (from 1 to 5).

From main activity I can call any of these subactivities and when I call the subactivity I am not finishing the main activity because I may need to comeback here at a later. **So the main activity is running in the background**

But when I enter the subactivity, I am making call. I am dispatching call activity to perform call operation. But **I want to kill all the activities except that "call activity**". the main activity is still in the background. How to kill it and all other activities which are active?