Apply strict network authentication and password policies to all VoIP users.

Encrypt all VoIP traffic and transmit it over a virtual private network where possible.

Configure firewalls properly and make sure that the underlying network infrastructure supports the session initiation protocol and the International Telecommunication Union’s H.323 voice protocol.

Limit the threat posed by packet-sniffing tools and minimise disruption in the event of an attack by separating voice and data traffic onto a virtual local area network.

Lock down server-based IP private branch exchanges and protect them against viruses and denial-of-service attacks using appropriate security tools.