The sip.conf file defines all the SIP protocol options for Asterisk. The authentication for endpoints, such as SIP phones and service providers, is also configured in this file. Asterisk uses the sip.conf file to determine which calls you are willing to accept and where those calls should go in relation to your dialplan. Many SIP-related options are configured in sip.conf, which was covered in depth in Appendix A, VoIP Channels.