A mobile number can only be linked to one STROOHM account. If you get this message, your number is already linked to another account, for example an account you created earlier with a different email address (private or work).
What can you do?
Do you know which other email address you have an account with? Log in to that account and change the mobile number via Me > Settings > Profile.
Then add the mobile number to the account you want to use and confirm it via the text message you receive.
Don't know which account your number is linked to, or it doesn't work? Contact the helpdesk and include your name, your mobile number and the email addresses you may have used.
Tip: only use your personal mobile number in your personal account, not in an account with your work email address.