For me, when playing/recording, it would really distract me (more than e.g. having a phone in silent mode so that when flow mode ends, I notice I'm hungry and need to wake up to the outside world, I can see who has tried to call).

The Kraftwerk description sounds a bit eccentric to me. Their business model is about being eccentric, so no surprise there.

>For me, when playing/recording, it would really distract me

I think this might be easier if you are working as a group and have a big clock on the wall. I am a rubbish instrumentalist, but do hang around with a hell of a lot of musicians, and 'how long was that' does seem to be one of the most commonly asked questions of each other when they are in a studio. Also, if paying for a studio, they generally become very good at estimating the current time.

Sure, but I think things like "we can now do one take and then it's phone call o'clock" would set an unnecessary pressure to the performance. That's just me of course.

Their manager can bang on the door?