Match your reprompting strategy to the situation
Consider using a “stepping stones and safety nets” approach to reprompting
If a caller has trouble with a concise initial prompt, try recovery with a “stepping stones and safety nets” strategy (Lewis, 2011). For example:

• System: Which best describes why you’re calling? Select New Claim, Life Policy, or Other Insurance Products.

If callers don’t respond to the prompt (noinput event) or say something the application can’t understand (nomatch event), then follow with messages such as:

• System: Please let us know why you’re calling. To report the death or disability of an insured, say New Claim (or press 1). For service or questions on an existing life insurance policy, say Life Policy (or press 2). For all other insurance products, including annuities and mutual funds, say Other Insurance Products (or press 3).

• Caller: <noinput, or nomatch event>

• System: To continue, please say New Claim (or press 1), Life Policy (2), or Other Insurance Products (3). You can also say Start Over, Operator or Goodbye.

With this approach, successful callers move quickly through the task (running across the stepping stones). For callers who are not successful with the concise initial prompting, the safety net pops open, with:

• Less concise, touchtone-style reprompting (see more detail in Chapter 9.)
• Presentation of global commands (as appropriate)
• Final concise reprompt (with possible switch to touchtone only at this point)

By analogy, concise initial prompting attempts to keep callers skipping across stepping stones because it isn’t possible to move quickly in a safety net. For a set of possible actions to take if the final attempt is unsuccessful, see What to do on failure.

We know of no comparative research on the many other possible designs to address this type of error recovery. For example, there might be cases when it would be better to present the global commands before the less concise reprompts. Some designers have a definite preference for a rapid reprompt followed by global commands (where appropriate), followed by the touchtone-style reprompting.

Be aware that depending on if or how you're using global error counters, a caller having repeated problems will hear the reference to the global commands a lot if this style is implemented across the board.

This is an area in which VUI designers should continue trying different approaches and publishing the results of their experiments. That said, the general strategy of stepping stones and safety nets seems to work well in practice.

Consider simply repeating the same thing for the very first prompt in a system
Sometimes the caller just simply isn't ready or listening. This is especially true if there's a lot of “stuff” up front before they get a chance to start interacting with the system.

Consider a “rapid reprompt” approach
The exact opposite of the stepping stones and safety net approach is rapid reprompt. This puts the reprompt out there as quickly and concisely as possible to give the caller another chance to respond. Sometimes it might simply be, “Sorry?”

This form of reprompting works best when the question is very simple and the caller is familiar with the system. If you have a system used by frequent users, they probably don't need more instruction, just another chance. Or consider a voice dialer. It is much more likely that the system simply didn't get the name right than that the caller needs more detailed instructions.

Lewis, J. R. (2011). Practical speech user interface design. Boca Raton, FL: CRC Press, Taylor & Francis Group