You must provide a mobile phone number and/or e-mail address when ordering a tax deduction card. With effect from 2018 onwards, this must be stipulated in Altinn under Profil>Din varslingsinformasjon for virksomheten (Profile > Your notifications for the enterprise).
You will then be notified by SMS and/or e-mail when one of your employees amends their tax deduction card. You must obtain amended tax deduction cards before your next payroll run.
If you obtain tax deduction cards from a payroll system, it is possible to turn off the notification function. Your payroll system supplier can advise you about that.