When a Siemens phone is busy, and a PSTN call arrives we get an "inband information" and "Cause= User Busy" back from Siemens on the qsig trunk and also send this "inband" busy back to PSTN, but the PSTN caller does not hear anything. So the busy tone is missing.
Within Siemens, everything works fine. Also between Siemens and Cisco. Also when a Cisco phone is busy a PSTN caller hear the busy tone. The only difference I see, is that the Siemens send us an inband information, and when a CUCM Phone is busy we have no inband information.