Managed Apple IDs (MAIDs) for users that have only one phone number?

We are rolling out MAIDs for all of our clients. However, we are running into a recurring issue where several users only have one phone number, and it's already being used for their personal Apple ID. Apple does not allow more than one phone number per Apple ID.

I have to assume this happens a lot, or will be happening a lot, and wondered what workarounds others have found for this. Currently, we have several users that cannot authenticate their MAIDs because they do not have a second phone number to use.

Thank you!