I would suggest that you normalize your tables and have an additional table for the telephone numbers.

What would happen if an employee has more than one cell phone? Or several extensions?

If you put them on the employee table as additional columns, you'll have to add enough columns to handle them. If a new type of phone comes up, you'll need to add a new column.

With a normalized table, there's no need to do changes to the schema, just add the needed rows to it.