Registering a SIP phone directly to Twilio allows for standards-based voice calls using a SIP endpoint, with flexibility to choose termination transport (PSTN, registered device, or Twilio Programmable Voice SDK endpoint). This enables features like IVR, speech collection, and intelligent routing decisions. To accomplish this, users need a Twilio account, a SIP client such as X-Lite, and E.164 formatted usernames for the SIP Domain registration credential list. The benefits of using Twilio SIP Domains include access to a large selection of standards-based endpoints, lower call leg costs, and flexibility in call routing logic. Two approaches are presented for outbound calls: TwiML Bins and Twilio Functions, with the latter offering more possibilities for dynamic call flows. Users can create a Twilio Function using NPM modules like google-libphonenumber to parse numbers and format them in E.164 format. The post also covers trial account restrictions, debugging tools, and future directions for SIP Domains and Twilio.