Thank you for your quick assistance and the information you provided.
I suggested this, because it seems easier. Despite this, a sipgate admin can impersonate it’s agents and check the URI part with the userId and communicate it to the Zammad admin.
Also - i think changing this, to use something different (e.g. the SIP-ID) as a replacement for the userId would break existing installtions for users.
Maybe this thread might help others, stumbling over this issue.