VOIP has taken off in recent years as a technology for communications, but the means of making it secure have had trouble keeping up. VOIP has known insecurities, related to such aspects as buffer overflows and packet header issues. Making a VOIP application secure can be important to an organization.
This link sets out nine different steps that can be taken to secure VOIP. The article also offers an excellent overview of the vulnerabilities of VOIP and how to address them.