A computer-implemented method for establishing a VOIP communication between a PBX system and a first terminal device is disclosed. The first terminal device has a VOIP application or video call application. The PBX system and the first terminal device are able to communicate via a first communication connection and a second communication connection. When the PBX system receives a call to be forwarded to the first terminal device the PBX system verifies whether the first terminal device is logged in. If the PBX system verifies that the first terminal device is not logged in, the call is rejected. If the PBX system verifies that the first terminal device is logged in, the PBX system determines whether the application on the first terminal device is running in a background mode or in a foreground mode. When the PBX system determines that the application is running in the foreground mode, the PBX system uses the first connection for forwarding the call. When the PBX system determines that the application is running in the background mode the PBX system uses the second connection to receive information about the communication connection being used.