There's no NEC code, but as nakulak mentioned ADA codes (if applicable) may come into play, especially for wall mounted telephones. If this is a new commercial building, pay close attention to the blueprints, otherwise you may wind up moving a whole lotta boxes.
Exactamundo. Other reasons aside, I like to make retro-work look like it's always been there. Match height, color, finish, etc., with existing devices.
Example: when adding a switch in a room for, say, a ceiling fan, you should change the existing box, adding a gang, rather than sticking a separate box.
Of course, if you're adding more than one switch, you might use a separate box, but you'd still match existing devices and plates, and height as well.