use the commands:
show dialer
show isdn active
show isdn history
they may not give u much, but they may get u started.. try the debug commands too...
u could just drop your config so we can look for what could be causing your line to come up.. if you've configured DDR, u may have allowed all IP to be interesting traffic, or u may have configured a routing protocol and not indicated your ISDN interface as passive..
it could be many things.. drop your config, and we'll help u out..
good luck..