To make the dummy user work we need to assign at least a Teams Shared Device License (or a full Teams & Teams Phone license). The Teams Shared Device license was known as a Common Area Phone license before.

Common area phones are associated with a user account that’s assigned a Teams Shared Device license. The Teams Shared Device subscription license includes a license for Microsoft Teams and a Teams Phone Standard add-on license.

Process to create Dummy users in MS Teams Adin portal : Click here.