A SIP domain is a custom DNS hostname that you create on Twilio that can accept SIP traffic for your account. If anyone makes a SIP request using that domain, say to sip:firstname.lastname@example.org, it will route to Twilio. Twilio accepts the incoming request and authenticates it using your chosen authentication method. If the request is authenticated, Twilio then requests your TwiML from the Voice URL you’ve specified, answers the call, and executes the instructions you reply with in TwiML.
All requests to different users on the same SIP domain are mapped to the same Voice URL; the request URI will be passed in the “To” parameter of the TwiML request. Additionally, some SIP-specific parameters, such as Call-Id, SipSourceIp address and any X- headers will be passed as well.
SIP Domain names are unique across all Twilio accounts and end in
.sip.twilio.com, for example, "example.sip.twilio.com". Domain names can contain letters, numbers, and “-”. By default, every account has their own Twilio Account SID reserved as a domain.
If you try and create a domain that already exists, you will receive an error. Creating subdomains of existing domains is permitted as long as you have already crea the original domain. For example: “foo.example.sip.twilio.com” can only be created if you have already reserved “example.twilio.com”.
Subaccounts can create subdomains of any SIP domain in the master account.