Hello, milk contains calcium, protein, but it is not irritating to the respiratory tract. What you said about phlegm has nothing to do with drinking pure milk.
Guiding opinions:
Phlegm is the secretion of respiratory tract. For example, catching a cold, having inflammation, smoking frequently and irritating the respiratory tract. Then there is chronic pharyngitis, so there is phlegm. Treatment is usually symptomatic, such as mucosolvan, a expectorant drug, and then anti-inflammatory drugs when necessary.