TAPI driver setup fails on Vista

dunno if this is a good place to ask but I have already asked for this in
the tapi groups and its still open. so here we go again:

I have a TSP (self-written, based on julmar) which has an installer (setup
project created with vs.net way back when). It used to work fine on all
win32 platforms. On Vista, it fails. Reason is that one of my custom
actions (which in fact is a DLL) calls TAPIs lineAddProvider .This fails
with OPERATIONFAILED (code 0x8...48).

I would assume that this has something to do with permissions, although the
setup is run be the local administrator.

Anyone with an idea what the reason could be and how to fix?

Thanks, Christoph