Many (if not most) VoIP devices have available a Web GUI for their configuration, management, and report generation. These Web GUIs are often on default, meaning that the moment you install the IP phone or IP PBX, the Web GUI is immediately available on the network. And unfortunately it is also common for the username and password to have the default values.
Sipflanker will help you find these SIP devices with potentially vulnerable Web GUIs in your network.
What the application does is search the range of IPs you specify, and checks if port 5060 is available. Whether open or close, port 5060 indicates the presence of a SIP device. Then it checks if port 80 (http) is open. The combination of an open port 80, together with port 5060, either open or closed, signals a SIP device with a Web GUI.