The normal lining of the lung clears phlegm by lifting it up and delivering it down the esophagus (you can think of this as
the food pipe).
The normal situation then is that,
phlegm is swallowed
in small, regulated amounts that it is entirely automatic (there is no need to push it down voluntarily in the manner of swallowing a lump of food).
Smoking destroys the lining, and hence to clear phlegm – the lungs must cough it out. Hence, the cough may be persistent.