Speaking engagement

The way in which one speaks has a great impact on chances of being hired. The phone screening, phone interview, and in-person interview are all indicators of job performance; if a candidate speaks inclearly or unprofessionally in these steps, what indication would the employer have the the candidate would do any differently on the job? A person must speak clearly and professionaly in all interviews and in the office. It's as simple as that. Slang is unacceptable. Swear words are unacceptable.