Well, actually I don't like any automatic updates and I'm always installing updates on any of my server manually, after couple weeks, from its publishing. After updating system, I'm checking if system works well, but as soon as I see that nothing was broken (at least it seems so 
), I'm forgetting about it. If the update is braking something, I always have backups to restore VMs to its previous state. But this bug is very specific and it took a long time to identify that we had the problem. Someone was calling to his colleague, he hears the ringing tone and no-one picks up, so eh thinks that the colleague is either busy or he's absent. That's it.
Anyway, I want to thank you, you really help me to clear my mind, as I didn't change anything in my configuration last couple months, I can't understand why it suddenly stopped working. Thanks one more time, now I'll try to find out more about downgrading Asterisk version and how secure it'll be for my platform.