Microsoft has not yet released a Teams client SDK/API for third party apps like Attendant Pro to make and control calls using the Teams desktop client. Because of this, an Attendant Pro user is required to use Skype for Business for calling. This configuration will leverage the Skype for Business and Teams interop to transfer calls to any Teams users.
Attendant Pro supports all interop scenarios and features that are supported by the Skype for Business client. See: https://docs.microsoft.com/en-us/microsoftteams/teams-and-skypeforbusiness-coexistence-and-interoperability