Auto populating a text field based on choice from drop down

I have a form and would like to be able to auto-populate a text field based on the user's choice from a drop-down menu.  I would want the text field to be populated with data that is currently in an excel spreadsheet.

So as an example.  Lets say the excel spreadsheet has a list of phone numbers.  If the user chose there company name from the drop down list, I would want the phone number filed to automatically populate with the number in the excel spreadsheet that corresponds with the company name chosen from the drop down menu.  What is the best way to do this?

Thanks!