Italian number - user warning: Data truncated

Once I create a new CCK entry for an Italian phone number, I receive the following warning message:

user warning: Data truncated for column 'field_phone_value' at row 1 query: ALTER TABLE content_type_venue CHANGE field_phone_value `field_phone_value` VARCHAR(255) NOT NULL in \htdocs\includes\database.mysql-common.inc on line 520.

If you need any specific information about my setup, please ask.

Andrew