Phones are ridiculously expensive! Some cost over $800 and still we don't protect them. Having a good case is important but it doesn't do much for you if you are trying to walk, job or bike and still listen to music. Those wires just don't stream out of your pocket well all the way to your ear without being jerked every time you move your arms.
The best place for your phone is on an armband that keeps it next to you so you can easily fiddle with it, check calls, and text. If you are not using bluetooth you can hook in a regular headphone set and have it run directly to your ear out of the way of your arms. Its a nice set up for someone who wants to be physically active.
Staying in shape can get a little boost when it is fun and you have music. It helps you take your mind off of your busy life and just enjoy the activity. Getting an armband for your phone will do wonders for your ability to stay active without stopping every few minutes. It should fit tightly and hold multiple types of phones. While your huffing and puffing have your phone handy.