1. In the import file, add a Phone Address Import ID column for each phone number being imported.
2. In the import file, move the AddrPhoneImpID column before the PhoneImpID column.  If there are multiple phone numbers being imported, do this for each "block" of phone numbers.
3. In the import file, the Phone Import ID must be unique.  If you are importing multiple phone numbers, make sure this ID is unique for each phone number type.  
4. Run the import with the updated import file. 

NOTE: if changes are made to the Import Field layout, you must refresh the field mapping.  
    1. Open the Import File defintiion
    2. On the Fields tab, click Clear Fields
    3. On the Fields tab, click Update fields
    4. If there are multiple phone numbers being imported, confirm that the extensions are mapped correctly.