I'd say it mostly depends on what you work on, and that is not "a field" or "a job". I've had jobs where yes, I could easily be on phone support during the tasks and I've had projects that were usually hours long deep thinking where you had to have a huge system + 10 classes in your head at the same time, but most jobs and projects were a mix. And then you simply may have a bad day and need deep thinking mode for relatively modest work, but yo won't notice the other way round if you build something great and bug free and it seemed easy.

I kinda don't believe the people who say they 100% have this job, all the time - but you also don't know beforehand when you need your interrupt-free time. Some days are just cursed and you get nothing done, even if they only interrupted you 3 times, but it was the important 15min block. And I've had those jobs as well. If someone (different people) interrupts you every 15min, when the day is cluttered up with 3 meetings anyway... I think it scales exponentially.