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If you need to call your boss to take off work, you need to sound the part to avoid suspicion.  Speak slightly slower. Pause occasionally for a second in the middle of your sentences. Don't be too quick to answer. Remember, you are sick and sluggish. Try to breathe through your mouth to sound like you have a stuffed up nose. Your boss might not care how you feel, but if you are going to get the rest of your coworkers sick that's another matter. Mention that you think you caught your illness from someone else. Explain that you are coughing or sneezing and your nose is running all over the place. Don't do this directly into the phone—you don't do that in real life, do you? Hold the phone a reasonable distance away from you and cough or sneeze loudly. Then apologize and continue the conversation. Pour one or two large glasses of water and make your call while sitting by the toilet. If you really need to sound sick, stop in the middle of talking to make a retching and gurgling noise and pour the glass of water. That should simulate the sounds of throwing up. One of the quickest ways to arouse suspicion is to ham it up. If you can just ask to take a sick day without giving too many details, you are less likely to get caught in your web of lies.
Make your voice sound different. Play up how contagious you are. Cough or sneeze. Fake the sounds of vomit. Don't overdo it.