Android Studio - Where can I see callstack while debugging an android app?

At the bottom panel you should have "5: Debug". Click on it and select "Debugger -> Threads"

You may need to find the "Threads" icon on the far right, or even click the "Restore Layout" button on the left to restore this window.

Seems like there is an UI-Bug in the Android Studio (1.x, 2.x and 3.x).

For me the "Frames/Threads" Panel was completely hidden behind the toolbar, so I had to change the size from the "variable" panel by dragging its left border to reveal the "Frames/Threads".

[I have to admit, that @Greg added this picture after reviewing my answer!]