If you experience failed faxes, running these commands can help identify if the issue is affecting a single modem/phone line or multiples modems/phone lines:
grep result $VSIFAX/spool/logs/modem1* | awk '{print $6}' | sort | uniq -c | sort -rn grep result $VSIFAX/spool/logs/modem2* | awk '{print $6}' | sort | uniq -c | sort -rn grep result $VSIFAX/spool/logs/modem3* | awk '{print $6}' | sort | uniq -c | sort -rn grep result $VSIFAX/spool/logs/modem4* | awk '{print $6}' | sort | uniq -c | sort -rn