In a migration from D6 using migrate 2.5 and migrate_extras 2.5 the following errors are thrown for every phone number record pulled from a cck field or a profile field when the field into which the data is put includes an extension.

3	Informational	Undefined index: extension File /home/dcogco5/public_html/dcogc.net/drupal7/sites/all/modules/cck_phone/cck_phone.module, line 525(file: /home/dcogco5/public_html/dcogc.net/drupal7/sites/all/modules/cck_phone/cck_phone.module, line 525)
3	Informational	Undefined index: number File /home/dcogco5/public_html/dcogc.net/drupal7/sites/all/modules/cck_phone/cck_phone.module, line 478(file: /home/dcogco5/public_html/dcogc.net/drupal7/sites/all/modules/cck_phone/cck_phone.module, line 478)

If the field is set not to have an extension only the second error message appears.

The phone numbers are migrated whether you sanitize them during the migration to remove all symbols or let cck_phone do that.