This chapter discussed many aspects of device state handling in Asterisk. We started by discussing the core concepts of device states and extension states, and built up from there. We covered how SIP phones can subscribe to states, tools for creating custom states, and two mechanisms that can be used for distributing states among many servers. Finally, we covered one of the features in Asterisk, Shared Line Appearances, that relies heavily on the device state infrastructure in Asterisk to operate.