I've been reviewing the Twilio Voice documentation, but I can't see if it would be possible for a sip communications infrastructure to join a Twilio Conference.
This would be the ideal scenario for us.
However, if this wasn't possible, having a conference dial or invite a sip address would be almost as good.
Obvious this is the basic sip dial from a pstn, but can't see how this could be used with the Twiml Conference verbs. https://www.twilio.com/docs/voice/twiml/sip
<Response>
<Dial>
<Sip>
sip:[email protected]
</Sip>
</Dial>
</Response>
Also I can see that the Api supports 'Create a Participant – Agent Conference only', however we don't use the Agent Conference Only conference type and we've concerned this would break existing functionality.
https://www.twilio.com/docs/voice/api/conference-participant-resource
I guess we're hoping this can be achieved with Twiml.
Since you are initiating a call out toward your SIP infrastructure (if I am understanding you correctly?), you would use the Twilio REST API Calls resource to do so, and the To: would be a SIP URI. The URL would be pointing toward TwiML which then join the SIP participant into the conference when they answer.
If you want them to dial into Twilio via SIP from your SIP infrastructure, you would set up a SIP Domain.
You actually have an incredible amount of flexibility with either approach.
Alan