Auto suggested fields

I think that the only way to do this would be to set up a table of commonly used phone numbers and then set up a relationship where one commonly use numbers has many of your main records.  Then you would also still need your existing data entry field.  Then you would need a formula field set to calculate the final phone number base on a formula that looks at both fields.  In view mode on the form you would only show the final field but an Edit or Add mode you would need to show the look up field as well as the data entry field.  And all your reports she would only use the final calculated field.