Did you ask this worker if he knew how to write business letters and use your phone system when you hired him? Was it on his Resume? If yes, than clearly you can fire him because of false resume. If not, then don't automatically expect him to know it, it's your job to train him.
Have you talked to him about your complaints and rants, or do you expect him to read your mind?
Anyways if he misrepresented his skills, or is not living up to the tasks that you explicitly outlined in the job description, then fire him.
If not, then you should have done your due dilligence when hiring him, and you are SOL.