Can my employer withhold my earned commission based on a “call quality” score?

The first issue is, what are the terms or agreement under which you work? If the terms were that your commission may be reduced according to "call quality," then this could be legal. However, if those are not the actual terms under which you work, the company may not reduce your commission for it unless and until they tell you those are are the terms--at which point you have the choice of accepting them or looking for a different job.

The second issue is, whatever the terms are, they have to apply correctly--so if, for example, your pay is subject to reduction for call quality, there must be documentation, some metric, some verifiable calculations, etc.--it can't be just at the whim of your employer.

Note that there terms under which you work can be found in oral or verbal agreements, or in the demonstrated course of dealing between you and your employer; it doesn't have to be in a written contract or agreement.

Long answer short: if applied with notice, consistently, and accurately, this policy may be legal. Without notice or applied arbitrarily, it probably is not. You should consult with an employment attorney, bringing all documentation, emails, etc. relating to your work, commission plan, how you've been paid, discussions regarding this issue, etc. and let the attorney evaluate your rights and options. Good luck.