asterisk users - Aug 2008 - Asterisk takes incoming call before extension was submitted

I have a Problem with incoming ISDN calls in Austria, I use zaptel and asterisk
bristuffed from Debian/Etch.

- If someone outside is dialing the phonenumber and the extension on an ISDN
phone, asterisk catches the call and puts into in the "s" extension
before the extension was submittet.
- The same Number dialed from a mobilephone works as expected, asterisk recieves
the extension. As well with "stored" numbers in ISDN phones.