One of the best things you can do to protect your VoIP phone system is to make sure the computers and other equipment you use to access this system are secure.  The most effective way to secure your system is to set up a SIP firewall.

SIP, which stands for Session Initiation Protocol, regulates voice packets as they pass between two endpoints on a network. A SIP-based firewall is able to inspect these voice packets as they pass through your network, and can filter out any traffic that looks suspicious.