We have built a product on Twilio that we sell to doctors offices. To my knowledge, what you're asking for is not possible as-is.

What you could do though: When someone calls, have the system text message you "Hey, this number called, click here to call them back" ... and the "click here" is a URL that hits your server and initiates the call from your server, creates a call conference, and then drops you and the person you're calling back into the call conference.