Hi guys,
so I did some more analysis on the logs as I found out that the dial application from Windows is doing it right.
So I found out that the TAPI application is disconnecting the channel after the transfer.
[2018-10-16 17:44:31] VERBOSE[17167] manager.c: Manager 'TAPI' from 192.168.139.175, hanging up channel: PJSIP/69-00002713
So I will contact the application vendor to check if there is something else to configure on their side.