When you send a text message through Google Voice, the text message appears to be sent from your Google number. When someone sends a text message to your Google number, and it's forwarded to your mobile phone, it won't appear as from the sender's actual number (e.g., the SMS may appear from 1-406-xxx-xxxx or 1-972-xxx-xxxx). This is so that when you reply to the 1-406-xxx-xxxx number or 1-972-xxx-xxxx number from your phone, the text message you send appears to be sent from your Google number and will be saved in your Google Voice inbox.
Taken from:
http://www.google.com/support/voice/bin/answer.py?hl=en&answer=117605
Sending text messages from a website like Skype or Facebook to your Google number currently won't be received. This is because these websites treat text messages as though they are sending a text message to an email address, which Google Voice doesn’t support. Also spam messages and certain message types that may not be supported by the mobile phone may not be forwarded using Google Voice.
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