From New York Magazine (via 99U): it takes just three seconds to break your concentration and make it hard to carry on with your task. So yes, sure, answering the phone is guaranteed to do that – but so is just hearing it ring.

Things like text messages, social media notifications, or a random email notice may be all it takes to distract you. Even if you don’t read the messages, check the notification, or open the email, as this new research shows: all it takes to break flow is a quick chime from your browser or buzz from your phone.

Even a 3-Second Distraction Can Screw You Up – Melissa Dahl, New York Magazine (14 May 2014)

Dahl’s article doesn’t say a huge amount more but it does reference the original research so check that out. And hat tip to 99U for spotting it all.