What could cause a phlegmy cough?

Infection usually: Any infection that causes irritation to the respiratory system will cause a productive cough with sputum. It is your body responding to the infection. Unless you are running fevers or it has been going on for more than two weeks you can probably just wait it out for it to heal.If it has lasted more than two weeks then see your dr. Or if you are running fevers have shortness of breath or pain.