https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/ is set up as a guide for just post-install to a minimal working SIP server. However, between the introductory page https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/The-Most-Basic-PBX/ and the phone configuration page https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/Registering-Phones-to-Asterisk/ there is clearly meant to be a section on SIP configuration for asterisk that is missing from the page set.
https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/ is set up as a guide for just post-install to a minimal working SIP server. However, between the introductory page https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/The-Most-Basic-PBX/ and the phone configuration page https://docs.asterisk.org/Deployment/Basic-PBX-Functionality/Registering-Phones-to-Asterisk/ there is clearly meant to be a section on SIP configuration for asterisk that is missing from the page set.