Historically Islands mode supported Microsoft Teams Direct Routing. Microsoft no longer supports Teams Direct Routing in Islands mode. It is not possible to migrate an account to Teams only mode with the service activated since the domains will not reactivate for voice.


https://docs.microsoft.com/en-us/microsoftteams/migration-interop-guidance-for-teams-with-skype

Phone System Direct Routing is not supported in Islands mode. 


Customers who are using the service with Islands mode, and wish to move to Teams Only mode need to follow the following process:


Note: this process is service affecting an will result in the inability to make and receive calls through the connector whilst the move is taking place, Due to the occasional unpredictability of Microsoft's activation, this process may take some time so is best performed during an off-peak time where a change-window of several hours is available.


The domains will be re-added to the tenant and provisioned for calling.

The transition is then complete