Birds sit on telephone lines because the high vantage point makes it easy to look for predators. A bird's natural inclination is to fly or perch, and telephone lines are convenient and easy to get to while being high enough that a bird's natural predators can't reach them.
Height also relates to authority or status with birds, so the higher the perch, the more status a bird has among other birds. That's why some birds will sit on the higher phone line, while others line up on the bottom line.
Birds do not get electrocuted on power lines because they do not touch both lines at the same time. If a bird does touch two different lines, it will cause a shock that kills the bird.