Ensure that PhoneImpID is included on the import spreadsheet, that all fields and extensions are mapped correctly on the Fields tab, and that, if the phone type does not already exist in the phone types table, Create new table entries is marked on the General tab of the import parameters.

Values in the CSV file for the PhoneType must match EXACTLY what is listed in Config>Tables>Phone Types.
If importing Email, it must be Email in Config>Tables>Phone Types (not E-mail, or email)