If you wish to add a mobile number on your e-Boks profile, in order to receive notifications, it is only possible to add a Danish mobile number to your profile if your e-Boks is Danish.
However, if your e-Boks is Swedish, then you are only able to add a Swedish phone number. The same goes for citizens in Norway who have a e-Boks. Norwegian citizens will only be able to add a Norwegian number to their profile, if they have a Norwegian e-Boks.
Unfortunately, it is not possible to register foreign numbers for one's e-Boks due to legislation.