Use this window for assigning a new phone number to an existing contact.  Simply select the desired contact, and click OK.

 

The grid can be altered using the common grid controls. In addition, you can type a name to auto-scroll to the desired contact.  Specifically, the grid will auto-scroll to the row where the typed characters match the first column.  To search using a different column, drag the column to the first position by clicking and dragging the column header.