And in case anyone hasn't mentioned it yet and you're here:
If someone can "brute force" your 6666 web user login, they can also attempt to register to a SIP account. If they succeed in that, they can dial out through your carrier(s) and spend your money. If you have an "auto-pay" set up with your carrier, that can result in thousands of dollars spent overnight.
This isn't a "never happens" scenario. We used to have clients come to us routinely after blowing $8000 on calls to a country they didn't even know they could dial.
So ... WHITELIST your systems. Vicibox 10 has an improved method of doing this. IPtables is installed in all linux servers and can be modified to whitelist pretty easily. If you have an older system, we published Dynamic Good Guys firewall system (free) over a decade ago to reduce the storm on this front.
It's a slight pain to configure a whitelist system, but much less than the alternative. Brute force attacks on web, FTP, SIP, IAX2, and ssh logins, even if they fail, can still result in DDoS-like downtime. WHITELIST.