Just be honest and say "please don't think I'm being rude, but I really need to get on with this work." You can be assertive without lying or being nasty.
That's ideal and needs to be the first approach. In practice, people that communicate all day don't always understand than other jobs require uninterrupted focus. That's why engineers try things like no-meeting-wednesdays and schedule blocks for coding.
I wouldn't use the tool in question, but I relate to the motivation.
if you need to use this tool, the uninterrupted focus is most likely already gone, damage done. it's even likely that the person will return once your call is done to interrupt you again.
or.... they'll learn that you prioritize answering calls over f2f, and they'll start calling you all the time. easier to 'avoid' to some degree, but you're just shifting the interruption.
By saying that you remind the interlocutor that being rude is in fact an option (indeed a widely used one) and you may simply be rude ... ah the subtleties of communication.
Can't speak for the others, but by merely saying "please don't think I'm being rude ..." you explicitly remind the other side that being rude is an option, and that what follows should not be taken literally. Indeed why would you say "don't think I'm being rude" except if you already anticipated that the other side will take it as being rude.
+1. I don't understand why we need a Chrome plugin to act like an adult. Or, on the flip side, why you can't be an adult and not distract a co-worker. ha
I wouldn't use the tool in question, but I relate to the motivation.