Your mouse is too high, to the side, or too far away.
Reaching for your mouse can lead to shoulder pain. Your mouse should be at approximately the same level as your keyboard, and positioned close to your keyboard so that you don't have to reach. You may also want to consider a touchpad or trackball to minimize the distance your shoulder has to travel. See Mouse Setup and Usage or Mouse Overview and Accessories.
Your keyboard is too low/high/far away.
This can cause you to raise or lower your shoulders to accommodate. Make sure your keyboard is positioned correctly (and that you're keeping your elbows close to your side as you type). See Keyboard Setup and Usage.
You're not maintaining good posture.
Proper posture is essential, and helps minimize the strain on your muscles and joints. See Keyboard Setup and Usage.
You're reaching too far to pick up or dial your phone.
If your phone isn't within easy reach, repeatedly extending your arm to grasp or dial it can lead to shoulder discomfort. See Phone Setup and Usage.
You're leaning forward to view the screen.
People who wear glasses often sit in awkward postures or lean forward to see properly, resulting in neck and back pain. Even those with naturally perfect vision sometimes adopt this position out of habit. Always ensure that you're using proper posture.