You need to make sure the users entering new employee names do not accidently enter the same employee twice. What can you do?
(The SSN field is the most likely candidate for a unique constraint because more than one employee may have the same first and last names, and if two employees live together, they would have the same phone number. The EmpID field is an identity field, so it is unique for each record already and therefore does not require a unique constraint. However, don’t count on it. SSN rules are riddled with exceptions—not to mention that people simply make them up.)