When adding or reviewing Users in the portal, you may see the following:
What It Means
The customer has an on-premise Active Directory and is using Microsoft DirSync to sync their users into their Office 365 Azure AD.
If any of those users have the msRTCSip-line attribute set in their on-premise Azure AD then we will be unable to apply a phone number using our background PowerShell script, until they have cleared this attribute and run another sync to Office 365 Azure AD.
If we detect this error when doing a Teams sync then we will show advice to this effect below the sync button.
However we also display that the user is DirSync'd, as shown above, to aid in troubleshooting and to help the customer avoid having aforementioned error.