You can check the following commands to assist in tshooting the problem
show isdn stat - The status should be:
layer 1 = active
layer 2 = MULTIPLE_FRAMES_ESTABLISHED
also check the following
show line - To display the parameters of the lines. Use the show line command to decide which lines need to be configured for dial in.
show users - To display the line number, connection name, idle time, and terminal location.
show diag - To determine the type of hardware installed in your router.
