A short list of some basic IOS debug commands used during some FXS troubleshooting and why...
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)
To save logs to the VG224, router, switch, etc., check the logging command and show logging to verify what is set now.
logging buffered debugging
Log debug messages to an internal buffer with a default size