Troubleshooting ISDN

Router#debug isdn q921	Displays info about Layer 2 (data link layer) access taking place on the D channel
Router#debug isdn q931	Displays info about Layer 3 (network layer) call setup and teardown between your router and the service provider switch
Router#debug dialer events	Displays messages when the DDR link has connected and what traffic caused it to connect
Router#debug dialer packets	Displays a message every time a packet is sent out the DDR interface

Note:

PPP is often used as an encapsulation method when working with ISDN. Therefore, the PPP configuration commands, along with the PPP debug commands are applicable here, too.