Logging of failed SIP calls (sipcmd) on a Linux box (Debian)

I have set up a little Raspberry Pi (with Debian 8) behind a router (Fritz!Box), which does check/analyse the connectivity or rather quality of service of a certain phone line per SIP calls. My phone line on the other side has an answering machine. Basically, I need to know, at which times the phone line is not reachable. In detail, the Raspberry calls said number three times a day, using the programme Sipcmd (https://github.com/tmakkonen/sipcmd) - see code below. Cronjob: 0 8,14,20 * * * /usr/bin/sipcmd -P sip -u abc -c cba -w 192.168.8.10 -x "c010101010101;ws45000;h" Now the calls work fine, but I need something like a logging into a text file, **when a call did not work or** rather, **when the phone number was not reachable**, so that it works like an alert system showing me only the failures. Anyone has got a solution for this?