Service provider revenues are under attack from all quarters—now add 411 directory assistance to the list. This week both Google and Microsoft rolled out free 411 services, or at least free until they eventually add locally targeted voice-based advertising to the mix.

Google made its offering available as a Google Labs project this week. Microsoft announced its offering Monday as part of a large update of its search offerings, although its voice 411 service isn’t live yet.

The new Google Voice Local Search is basically 411 service delivered for free. Customers dial 1-800-GOOG-411 and interact with a voice recognition system. Right now, the service is focused on helping customers find businesses, either by name or description. Once a customer finds the business they want, Google will connect them for free or send a text message with details.

Since the service is still in Google Labs, it doesn’t include advertising. But long term, it fits into Google’s vision of extending its ad reach as broadly as possible, including to services that tread on traditional telco business.

Microsoft, meanwhile, announced Live Search 411 this week. Jointly developed with Tellme Networks, which was acquired by Microsoft earlier this year, the service is available via 1-800-CALL-411. Like Google’s offering, customers call the number, track down a business and can either get connected at no charge or receive a text message in return.

Google and Microsoft aren’t the first companies to roll out free 411 services, though obviously their size and scope automatically make them forces to be reckoned with.

For instance, startup 800-FREE-411 offers a similar service and already has targeted advertising built into the service to help fund it. The service also covers residential and government listings (Google 411 only delivers business information) and offers a live operator backup in case the voice recognition system can’t deliver a match.