When employment starts, so does the employer_employee relationship. This type of relationship is defined by the kind of contract the employee has in the company. It revolves around the things they talk about and disclose to each other before the start of the employment. And before such employment, it is necessary that the company has certain basic information about the employee. This is basically for accountability purposes and for the sake of having a complete and uniform record of all employees.
A template for an employee form could also contain a portion asking for the employee's office extension number. This would probably be part of the form especially in big companies where an employee may be reached by another in a different branch office through phone.