And very good advice it is too, especially when travelling at a steady speed on motorways and the like, where there's a constant whine of background motion-noise and you have to raise your speaking volume to be heard above it. This is insidious because after awhile your ears become accustomed to the increase and "tune it out" so it falls below conscious awareness, and you don't realise the strain it's putting on your vocal cords. I remember being warned about this years ago by my voice teachers. The same can apply to phones, especially if you're outside or in an environment where there are competing sounds.