If in the pop-up "Acquiring Microsoft Access Token - invalid_client " is seen, then this indicates that the Microsoft Office 365 tenant is not fully setup, or the setup has failed.
Reason 1: Office 365 internal setup has failed
On rare occasions the Microsoft Office 365 system orchestration can fail and provision services out of sequence. This leads to a lock-state on some of the elements within the Office 365 permissions database. This situation has occurred when Teams is under heavy load due to extraordinary circumstances.
The Azure permissions for Call2Teams (or relevant brand application) have to be removed and the setup run again.