Every hour, get up, walk around the desk and around the room. Bend over, touch your toes and stretch a bit. If you need a more comfortable chair, ask for one or, buy a gel cushion to put on it. As for keeping occupied, you aren't supposed to be doing anything except your job. If some of the time there is nothing to do, straighten your desk, organize the things on it, in the drawers. If all is well, tell your manager and ask if there is something he can think of that needs doing that you can do when you are not answering phones. In other words, let him know that you have nothing to do some of the time so that he can find something to keep you busy. If you start doing your own thing - personal things, you will likely be fired so I do not recommend that.
Certified Paralegal, with 25+ years' experience & with Employment law experience.