GZ,
Doing a bit of research on this topic, it appears that the callback busy feature is not supported over SIP trunk at this time. This is not exclusive to Siemens. It is also true when SIP integrating with Avaya, Nortel and Alcatel. Each of these application notes includes the following comment in the 'limitations' section: "Call Completion (Callback) Feature is not supported on either system using standard SIP protocol"
Cisco does support CBB over QSIG links to most major PBX's, so at this time, if you require CBB support, you would need to implement a QSIG trunk to get that level of integration.
Thanks,
Dan Keller
Technical Marketing Engineer