It's most likely a config issue w/
the ISDN router.

I used to use ISDN to get office
connectivity and it had to be
config'd so that it would not
'dial on broadcast'.

There were other packet types such
as ARP (or was it RARP?) that would
cause dialing out.

You need to look at the ISDN manual.

I used an Ascend ISDN router which
offered alot of flexibility.

You probably want to use a packet
sniffer to look at what is being
put on the wire.