Want in on Google Voice’s web-based, transcribed, custom-greeted voicemail, but you’re not quite ready to adopt a new number? Starting tonight, Voice users can choose to keep their number and still get Google’s upgraded voicemail features. (Note that this still requires a Google Voice invite as GV is still in beta).

Google Voice is a Swiss Army knife of cool and free phone service add-ons — including free SMSes, an online mailbox for voice messages, the ability to have one number ring all of your phone numbers simultaneously, low international rates and a customized voicemail messages for every contact. It’s not phone service per se though, since you still need a mobile phone or landline.

But using Google Voice requires users to use their Google Voice number as their main number. That’s a not-inconsiderable burden, given that some mobile phone users have thousands of contacts who know their number and don’t want the hassle of changing business cards and forcing others to update their contacts.

Google’s solution? Create a light version that gives phone-number-huggers better voicemail. Using a mobile carrier’s call-forwarding codes, Google Voice Light will send a mobile phone’s unanswered calls to a Google-powered mailbox. When callers leave a message there, Google records and transcribes it, and saves it in an online mailbox. The roughly translated text and a link to an online recording can be sent via SMS or e-mail.

The capability will also benefit those who have migrated to Google Voice, since currently the voicemail feature only kicks in when people call the Google Voice number, which forwards the call to a user’s mobile phone. Currently, those who call the mobile phone directly leave a message using the mobile carrier’s network, but with the new system, those calls can be diverted as well.

The voice messages can be stored in perpetuity, forwarded to family or friends, and they can be saved, even if you decide to switch mobile carriers. In return, Google gets your loyalty, more users with Google accounts and more pages for it to place online ads. That’s also not including the training data it gets for its translation engine — not dissimilar to why Google offers a free phone number lookup: GOOG-411.

This doesn’t really help me since my carrier charges for forwarded calls… bummer.