VoIP attack, although not an everyday occurrence, itdoes exist. When using VoIP, system security is undoubtedly one of the issues we care about most. With appropriate configuration, and some basic safety habits, we can improve the security of the telephone system. Moreover, the powerful built-in firewall function in Yeastar S-Series VoIP PBX is adequate to enable the system to run safely and steadily. We strongly recommend that you configure firewall and other security options to prevent the attack fraud and the system failure or calls loss.