Accept random length DTMF keys using VoiceXML

I am developing a dtmf-only application, and I tried to use <grammar mode=”dtmf”> but it didn’t work, so I’m using now and it works fine. But there are some dialogues, where the user has to enter some id or some number of random length. I tried to use built-in “digits” grammar as: <field name=”lala” type=”digits”>but it didn’t work.

You can use <dtmf>$digits</dtmf>

Users are expected to enter digits followed by #