How do I get rid of phlegm COMPLETELY?

You should try those "neti pot" or other nasal rinses. Or if that's too much, just some plain saline nose sprays. It will help clear the sinuses and the plegm that sits at the top of your throat. Also you may want to try guafenissen tablets. They sell an extended release brand Mucinex, but there are also regular tablets that are much cheaper. It kind of thins the mucous, making it easier to cough up. Also, you can never get rid of phlegm completely.