Heh. I know what SIP is. I've done a lot of telco programming. What I meant was, in what way do you want twilio to support sip? I'm sure it's what they're using on the backend, with a sip trunking provider (or multiple)...

Is there a reason to want them to send SIP rather than calls over the PSTN? If you want SIP, using a hosted solution may not be the answer...