Flights should be like movie theaters when it comes to using your phone. Don't spoil the ambiance for everyone with your chatter. A message is fine. But for voice, kindly step out.
Step out of the plane ? :)
More seriously, if people have to move to make a call, this will cause a mess in the economy section, where seats are narrow and you need to step on people's feet to move around. And that will make the job of air hostesses way harder when it comes to moving around with their carts, too.