Can I playback a TT String that was entered, playback, and then give the opportunity to accept or re-enter?

Of course! The Get TT String step requires that you supply a variable to capture the Touch Tones. After the Get TT String step, you can use the Say step to playback the digits entered. On the properties for the Say step, set the "Format to speak" field to "Digits", and the "Value to speak" field to the variable used to capture the TT String.