A list of some IOS debug commands I’ve used during some FXO/FXS troubleshooting

debug mgcp [packets]
Use to view the complete MGCP packets exchanged between CUCM and router.

debug vpm signal
Use to view the on−hook and off−hook signaling for the voice port.

debug vtsp dsp
Use to view digits collection performed on the router.

debug vtsp tone
Use to view any tone generated by the router (dial tone, busy signal, fastbusy, etc)