ekanes--Have you looked into setting these up like freeconferencecall.com where it dead-ends the calls out of a rural area so you can collect subsidies? (Not a fan of the legislation, but if it exists you can leverage it.)

Not sure how you'd do this using Twilio, but rather than routing the user straight to the number, you could route the user to the rural number and then place an outbound call from teh rural conference number and patch the two together