I found it more useful to control my Twillio interaction via a Finite State Transducer (FST - http://en.wikipedia.org/wiki/Finite_state_transducer) instead of a FSM.

An FST allows you to accurately model 'outputs' on state transitions (where those outputs can be side-effects like 'make a call' or 'send an invoice.' This allows you to prove things about what sort of side-effects/outputs will necessarily have occurred by the time the FST is in a certain state.

I started with an FSM but, found in practice, I was using it like a poorly written FST.