Can I link my phone number to multiple private and business accounts?

A phone number can only be linked to one bank account at a time. If you want to link a phone number to your BNP Paribas Fortis account, but it's already linked to an account at another bank, you'll need to remove it from the other bank's account first. Only then can you link it to your BNP Paribas Fortis account.
If you have both a personal and a business account, keep in mind that you can only link your phone number to one of these accounts.