Bulk import attempt fails csv contains voicemail_enable yes and does not contain a column called voicemail with the context noted.
GUI throws error: undefined index: Voicemail FIle:/var/www/admin/modules/voicemail/Voicemail.class.php:1737
once the error is displayed, the import button is greyed and not clickable.
Adding a column called voicemail with a value of default and reimporting allows the import to succeed and the extensions have enabled voicemail.
The bulk export of extensions does not contain the voicemail column nor does the import page indicate it to be a required column.