Wung I had this problem all of a sudden, the isdn device replys on the D channel it was connected, then- no data on the main channel,, so ISP dropped channel, It was a LUCENT DEFINITY switch, local telephone exchange in the Office Bld with ISDN simulator cards. we tried everything,, Virus checks, different PC's etc etc, the service agent did a days worth of tests.ISDN Phone handset worked ok.still the same. swopped over to a BrTelecom fixed ISDN line all ok now. i suspected a software "upgrade" which downgraded the operation