When you do radio interviews from your office you have that luxury. When you sit, often the tendency is to slump and take short breathes. When you stand, you can breath more deeply and project from the diaphragm. Remember to take a deep breath before you begin speaking. Holding the telephone or cradling the phone on your shoulder for an hour can cause stress to your neck.
Many professional speakers use their hands when they speak to emphasis points.
Using a headset allows you to act and speak more naturally. Tell the truth. Hosts value real experiences so use some stories from your own life to embellish your message. If you have experienced a personal struggle or triumph, tell the story and be sure that it is relavent to the topic you are there to talk about. Hosts also value brevity.