Assigning Microsoft 365 Phone System License fails with strange/confusing error message

Brass Contributor



for internal tests we currently assign the Microsoft Phone 365 License via the Admin Portal manually. All other Licenses are assigned automatically via synced AD Groups.


If we now assign the Microsoft 365 Phone System License by hand via we get the error message (Names replaced, tough username@contoso is exactly the same). 


The email address is being used as an alternative email address by user (Lastname, Firstname) Use a different email address.


We can't add the license via Powershell or AzureAD. In Powershell nothing happens, though the commandlet runs successfully. In AzureAD -> Licenses their is also just a  generic error. 


On some users it works on some not. Their is now visual difference eg. with the upn etc.


It is also the case with the Audio Conferencing License. 

We have done this many times before with Audio Conferencing and for a couple of users with Phone System  without any problems. 

We have no clue and it seems MS Support also not :(

any hints ?


Many Greetings




1 Reply

Late reply, but for those who stumbled upon this thread I just had this issue today, same error message. Issue turned out not to be with the email address listed in the error, but a separate duplicate email address associated with the stated account. So really there is a problem in error handling. To summarize the issue and fix...

*Error states "<EmailAddress>" is a duplicate on "<Person1>". The person was correct, the email address was incorrect.
*"<Person1>" had "<DuplicateEmailAddress>" listed as an alias
*"<Person2>" had "<DuplicateEmailAddress>" as a value for their mail attribute in AD.

Removing the "<DuplicateEmailAddress>" from "<Person2>" mail attribute in AD and performing a delta sync resolved the problem