asterisk users - Sep 2004 - isdn, pbx and *

i have a traditional pbx attached to one line of NT1, and asterisk with hfc-s 
to the other one;
when a call comes in, it is like asterisk captures it, passing it to the 
channel configured on dialplan;  in the facts, the call is not answered, but 
NT1 doesn't ring pbx, as it would do if call had been answered.
What is happening?
Can i avoid this and have pbx and asterisk ringing simultaneously?
	10x, Maurizio