Google on Thursday began issuing invitations to users who have expressed interest in Google Voice, the company's online voice mail and call management system.

"We are happy to share that Google Voice is beginning to open up beyond former GrandCentral users," said Google Voice product managers Craig Walker and Vincent Paquet in a blog post. "If you requested an invitation on the Google Voice site or previously on GrandCentral, keep your eye out for an invite e-mail."

Google in May reportedly reserved 1 million phone numbers through network operator Level 3 Communications, a move seen as a sign of the company's intent to open Google Voice to a large number of new users.

Google introduced Google Voice as a private beta test in March and said at the time that the service would start accepting new users in a few weeks. The service was acquired by Google in July, 2007, when it was known as Grand Central.

Google Voice provides a new phone number that can be used to route calls to up to six other phone numbers, based on criteria determined by the user. It also features an online voice mail inbox that stores calls and provides speech-to-text transcriptions.

It's most broadly appealing feature is the ability to place free domestic calls in the United States and low-rate international calls to select countries.

Other features include the ability to screen calls, to block calls, and to receive e-mail or SMS notifications when voice mail messages are left.

Google Voice allows users to personalize greeting messages and the ringing sound that callers hear while awaiting an answer, and to determine which phone numbers ring when a call comes in.

And it supports a WebCall button: When embedded on a Web page, would-be callers can click on the button to initiate a call.

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