ppp through external isdn device

I can't get a successfull conection with an external isdn device, sometimes negotiation comes along, I get an IP address and DNS but am dropped down little after. This is aht the logs look like: May 23 14:01:33 devserver pppd[3341]: pppd 2.3.11 started by root, uid 0 May 23 14:01:34 devs

External ISDN device

I have to connect a linux box to the Internet through an external ISDN device. I have read the ISDN4Linux stuff but everything there is about internal devices. Should it be treated as a normal modem connection using pppd? Do I need task-dialup-isdn at all