You need to collect exactly one digit from the Get Digit String; then (assuming one was entered) incrememnt a counter. Then have another Get Digit String step. If that one collects a digit, then you're dealing with an extension (keep collecting digits), otherwise if collectedDigits=1 then it's a menu option and you can determine what to do based on what that digit was.
You can look at the attached example. It does not handle extensions that overlap with menu choices (see the README), but you could do that, too, if you wanted to.
