How to import data into the homePhone attribute within AD with a CSV file without overwriting the existing value.

Hoping you guys can help. I have been given a list of over 2000 users as a CSV file with 2 columns, "employeeNumber" and the corresponding "homePhone". Both only have numerical values. The management want me to update the users homePhone attribute within AD from the list provided. The catch is not to update the field if it already has a value. So, in essence I only want to update the AD accounts that have a blank attribute. I am hoping to use the import function within Powershell. Thank you.