State of knowledge
Since the update from Windows 10 to version 1903, the Bluetooth connection of smartphones to the ProCall client no longer functions.
Since the latest Windows versions, a separate handsfree driver is supplied, which is related to the mobile phone.
This seems to have priority over the handsfree driver of ProCall.
As of ProCall 7 Enterprise SR1 (7.1), the ProCall Client for Windows automatically deactivates the Microsoft Handsfree driver so that the driver Bluetooth Handsfree Audio Gateway from estos GmbH can be used again.
This behavior can also be set manually in the client settings under Bluetooth.
Current workarounds with restrictions for versions < ProCall Enterprise 7.1
The following workarounds were developed:
Disable handsfree driver
Disable the Hands-Free driver in the Device Manager → System Devices → <Handyname> Hands-Free driver, that is directly related to the mobile phone, start the ProCall client again.
Open the Windows Control Panel and go to Devices and Printers -> Properties of the <Bluetooth Device> -> Tab: Services
Deactivate the "Hands-free telephony gateway" service.