Most malicious individuals utilize open network ports to break into VoIP systems. They look for anything from weak passwords to known software bugs and attempt to exploit those setups to control the configuration and routing of a phone system. The general goal is to commit fraud, eavesdrop on calls, or steal information (such as voicemail messages).
Since the network is the entry point to your system, it's important to pay close attention to how your network is setup and take advantage of some of the functionality within FreeSWITCH to secure your system further.
SIP is a technology that is commonly targeted for abuse on the open Internet. In most cases, malicious hackers will attempt to scan a range of IP addresses by sending UDP packets on port 5060 and look for servers that respond. Once they find a server which responds, they will attempt to brute-force common passwords or simply try to dial out. In some cases they will...