Is it possible to make a call without first registering?

I was playing around and learning. I opened port 5060 on my internet facing router to do some experiments.

A day later, I found thousands of SIP connection attempts per hour (all Wrong password). But Messages in the log folder also show things like:

Does it mean that someone from was able to initiate a call attempt, although it was not successful?

I can account for all in Master.csv except:

I have no extension 1236. What does this CDR mean?

I must now go and check my IDD bill!

The message is very clear

add this setting to your sip.conf file in the general section.