Just my 2 cents:
I've encountered a bot, who sent random INVITEs and never answered to 401 responses. My console was really FULL of this mess.
I banned him forever using this recipe: http://www.mauromascia.com/en/blog/fail ... an-per-ip/
Somehow, this was not caught by my fail2ban asterisk logs watcher. Would/Should it be possible somehow [to make asterisk write WARNING messages with IPs of non-responsive 401 receivers] ?