Hello,
hey I am also from belgium! ;)
Have you tried disabling the "allow anonymous incoming calls"?
I guess someone is trying to scan for valid extensions to later use it to call using your credit..
also, it's good to use fail2ban, I have many asterisk PBX, if you don't secure them thoroughly they quickly get hacked and it can cost you thousands of euro..
Also, don't rely on the CDR to debug this kind of issue, as the CDR can fail at many levels, you should check the logs in /var/log and leave an asterisk CLI connected..
asterisk -rcvvvvvvv
I am looking forward to help you fix this issue.