Google Voice (previously GrandCentral) gives you one number for all your phones — a phone number that is not to a device or a location. It offers additional features like voicemail transcripts, call screening and blocking, conference calling and more.
Google Voice App for Mobile is a new software that allows you to use your Blackberry or Android-powered device to place calls and send SMS messages from your Google Voice phone number. It fully integrates with the mobile phone address book to make calling and messaging easier.

Key features of Google Voice Mobile App for Android and Blackberry include:

Make calls and send SMS from your Google Voice number
Save money with low priced international calls
Listen to your voicemail and read voicemail transcripts
Manage your Inbox and view Call History

On a Google Android phone, you can use the Inbox to listen to your voicemail messages and view text transcriptions. From the Inbox you can read a text transcription of a voicemail, play a recording of the message, or even add someone to your Contacts list. You can respond to any of the messages in your Inbox by placing a call or sending a text message. If you select Labels, under Menu, you can jump directly to any of the following sections:

Starred
Voicemail
SMS
Recorded (calls)
Placed (calls)
Received (calls)
Missed (calls)

You can also choose whether to use Google Voice for all calls or only for international calls.

On a Blackberry device, you can access your recent voicemail transcriptions, message recordings, and SMS messages through the Messages tab, and use the Dialer tab to place outbound calls.

If you don’t have an Android powered device or a Blackberry you can use Google Voice on your phone by visiting www.google.com/voice/m.