<grammar mode="voice" root="passward">
         <rule id="digit">
          <one-of>
            <item>0</item>
            <item>1</item>
            <item>2</item>
            <item>3</item>
            <item>4</item>
            <item>5</item>
            <item>6</item>
            <item>7</item>
            <item>8</item>
            <item>9</item>
          </one-of>
         </rule>
         <rule id="passward" scope="public">
          <item repeat="6"><ruleref uri="#digit"/></item>
         </rule>
        </grammar>

<form> <!-- If nothing else is specified, the first <form> that is encountered
   is used as initial dialog -->  
 <field name="ticket_num">
  <prompt>
     Read the 12 digit number from your ticket.
  </prompt>   
   <filled>
     <if cond="ticket_num.length != 12">
       <prompt>
          Sorry, I didn't hear exactly 12 digits.
       </prompt>
       <assign name="ticket_num" expr="undefined"/>
     <else/>
       <prompt>
          I heard <say-as interpret-as="vxml:digits">
          value expr="ticket_num"/></say-as>
       </prompt>
     </if>
  </filled>
 </field>
</form>
</vxml>
openvxi不支持，没有built-in grammar，
后来用srgs，发现repeate根本没有反应，我输入一个串，发现说部认识，
如果输入一个字符就好的，望高手指点一下。