[...] When you started the phone call, you had to think about it, find the phone number, and make the call. When you were interrupted, you had to figure out what the person wanted from you in order to be able to give them a decision. Finally, when you resumed the phone call, you had to remember where you left off. You might even have had to say, "Oops, sorry, someone walked in. What were you saying?"
The more starts and stops you make during the day, the more of these start-up moments you have. These moments are non-productive time. If you have a daily report to prepare, the start-up moment is probably pretty short compared to that for the report you only do quarterly. Still, if you are preparing that report and you get interrupted, you have almost the same length of time for start-up each time.
I can write my weekly report in about 30 minutes without any interruptions. I have had it take several hours to complete simply because my job is operational and requires that I deal with unplanned tasks a lot. The culprit was not the people who interrupted me. The culprit was the time required for start-up moments each time I re-started the report.