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.
You can find a list of default IP phones and other SIP devices here.