Netbios keping routing connection alive?

We have a network which has dual ISDN through our ISP we also have a router of type AR130.
Problem is the router wont time out the ISDN when not in use so we are paying for an ISDN line constantly.

Any idea which services could be causing this and what we could do to eliminate the problem.
We have reset the router with the line set ppp=0 over=ISDN(Ourname) idle=180

Have trawled the net and some say it could it be bindings or Netbios but dont give anyway to resolve.
Any help very much appreciated.