Asterisk with Zyxel P-2002

Does someone tried to conect a Zyxel P-2002 point to point configuration using Asterisk like SIP server?

I’m trying to run my Trixbox with my P 2002 L from Zyxel. It is working fine, but there ist one little problem…

If a client (eq. a softphone) calls the 2002 via an asterisk it rings only two times and then the 2002 answers with an 200 OK and the RTP session is opened although nobody has answered?!?!!

Before the 2002 answers it does a lot of DNS querys. maybe there is a network problem…

if you have the same problem, please report!