Twilio’s SIP Domains are a SIP interface to a TwiML application. If you want to send voice traffic to Twilio via SIP to interact with your TwiML Application, then SIP Domains are right for you.
Elastic SIP Trunking provides a streamlined cloud based SIP Trunk at very low prices. It offers both call origination & termination. If you have a need to connect your Business to the PSTN via a SIP Trunk, then Elastic SIP Trunk is right for you.