The tasks in this sense are the major ones that require near full-concentration.
Walking and chewing are not really tasks.
Tracking the time and updating the mouse pointer aren't really tasks.
To convert from a computer view to the human view is to imagine one computing second or modest fraction thereof as equal to an 8-hour workday.
Zip, a day goes by. The organic unit has answered x calls, and made Y progress on the two major unrelated projects in progress.
Context switching takes a few seconds of that 8-hour day... switching to the phone isn't noticed much because of the automatic handshaking process when you pick up "hello... hello" that covers your switching time.
Then when you switch back from the call, you sit for a second or two going "ok, now where was I... ah."
Plus if you get overloaded with switches, you'll overflow your mental stack and completely forget what you were originally doing before the interruptions started.
