Twilio’s SIP Interfaces are a SIP interface to a TwiML application. If you want to send or receive voice traffic to/from Twilio via SIP to interact with your TwiML Application, then SIP Interfaces are right for you. SIP Interfaces also allow you to directly Register your SIP devices to Twilio.
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 Phone System (PBX) to the PSTN via a SIP Trunk, then Elastic SIP Trunk is right for you.