Every time my phone rings…

... it costs me at least 15 minutes of productivity. Possibly more.

The trouble is, getting into "the zone" is not easy. When you try to measure it, it looks like it takes an average of 15 minutes to start working at maximum productivity.
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The other trouble is that it's so easy to get knocked out of the zone. Noise, phone calls, going out for lunch, having to drive 5 minutes to Starbucks for coffee, and interruptions by coworkers -- ESPECIALLY interruptions by coworkers -- all knock you out of the zone. If you take a 1 minute interruption by a coworker asking you a question, and this knocks out your concentration enough that it takes you half an hour to get productive again, your overall productivity is in serious trouble.

Some people wonder why I hate the phone. well, that's why. It's so easy to call people, and very hard to ignore. And before anybody says anything, no it's not that easy to remember to turn down/off the ringer before you get into the zone. Sometimes that just happens: the rest of the world be damned.