Based on your account that is configured on your Enterprise's Call Manager, MiCollab for Microsoft Client enables you to specify the device used for outgoing calls. The Call Using menu specifies the device used for outgoing calls.

To set the preferred device for outgoing calls:

Right-click the Client icon in the system tray.
Hover over the Call Using menu and select the preferred device for outgoing calls.
Note: The list of devices displayed are softphone, deskphone, and mobile number depending on your account. Contact your system administrator to know more details on the list of devices configured.
Click Done to save the settings.

The device setting can also be accessed from Preferences > Call Settings > Call Using.