No, benzonatate (generic for Tessalon) will not loosen mucus. It acts only as a cough suppressant. If you are looking for a medication to help clear congestion and mucus, a drug like guaifenesin would be more appropriate. Additionally, to treat sinus pressure, a nasal decongestant like pseudoephedrine would be an option that is often recommended.
Benzonatate works only to suppress cough simply by acting as a local anesthetic in the airway and lungs. Specifically, it anesthetizes (i.e. numbs) stretch receptors (i.e. pulmonary stretch receptors), which are involved in the reflex control of breathing and cough.
The prescribing information for Tessalon states this mechanism of action as follows:
TESSALON [benzonatate] acts peripherally by anesthetizing the stretch receptors located in the respiratory passages, lungs, and pleura by dampening their activity and thereby reducing the cough reflex at its source.
Now, since you are concerned about loosening and removing excess mucus, I want to be sure to point out that taking benzonatate can actually make this more difficult. The action of coughing itself helps to remove mucus and phlegm. This is also known as expectorating. If you suppress cough, clearly you won't be clearing mucus as effectively.
As I mentioned at the beginning of this answer if you are looking specifically for a drug to help you thin and loosen mucus, guaifenesin may help, although it generally is only slightly more effective than a placebo according to several large studies.
One of the best things you can do to loosen mucus is to drink a lot of fluids. This, on its own, with thin mucus, making it easier for our body to remove.
SummaryBenzonatate works only as a cough suppressant and will not help to loosen mucus. In fact, it can make it more difficult to remove mucus since expectorating (i.e. coughing up phlegm and mucus) will be suppressed with benzonatate.