I'm going to sound like a tyrant here, but I think mobile phones are the bane of a well-oiled production, as "it's not the time it takes to take the takes that the time; it's the time to talk between the takes that takes the time." My feeling on a set is that all attention should be focused on making sure you have as much rolling-camera time as possible and to facilitate that you need everyone doing their job quickly and efficiently between setups, and that means the extra 30 seconds it takes Joe to get back to the set because he's texting or calling his buddy is 30 seconds you're not rolling camera. Multiply that by the size of the crew and you're talking losing several takes a day at least.
I've noticed that on shoots where the cell phones OFF rule has been applied we get more shot in the same amount of time.