ZDNet has an aritcle on rising attacks on VoIP systems. Most of the VoIP attackers seem to employ the method of spamming UDP port 5060. SANs Institute have reported recent spikes in SIP Attacks,) Session Initiation Protocol (SIP) scanning).
We wrote about SIPVcious before, use it to see how strong your systems are.
Even with best security practices, if your clients are using easy to guess passwords, could allow hackers to compromise your system and send your call costs hundreds of thousands of dollars, say during a weekend.
There are many simple little things you could do to limit the damage from these attacks, like changing the SIP ports from the default ones.
Also do not be complaisant just because your server is behind a firewall. Write up some rules for you firewall or like us use some tools like fail2ban to block port scanners. Utilize Blacklist from VoIP Blacklist Project to secure your systems. FreeSWITCH has the script in their source tree.
What ever you do, work a bit hard and keep those hard earned savings in your pocket. We will be posting an article on Asterisk-fail2ban, as soon as Diordna returns from his travels.
ZDNet via FreeSWITCH