If you try calling 1-800-BING-411, you will hear a recording telling you the service is going offline June 1st.
I have no clue how many people actually use the service, but I bet less and less every day.
With all these smart phones with embedded voice search native to the device (Google Voice Search & Apple Siri) there is less of a need to dial a phone number to get voice search.
Forum discussion at Google Blogoscoped Forums.
Postscript: Bing has extended this service until July 1st to give their customers more time to transition elsewhere.