"Display my Google Voice number" Is Your Choice Now.

I have always wondered, silently when ever I got an SMS message from Google Voice, "why do I have so many messages from G0O? (406 on your keypad)"
But I learned the reason today;

you may have noticed that the message does not appear to come from the sender's actual number, but from a (406) number (which spells "GOO" btw), with the sender's number (or name if they are in your address book) at the start of the message. We do this so you can reply from any of your phones and keep a record of the entire conversation online in your Google Voice inbox.

But this get complicated when you send an SMS from one of the phones on the Google Voice, to another Google Voice user. Instead of 406 number combination, Google Voice has automatically sent your Google Voice number with the message.
But Google Voice has decided that ultimately it is you who should decide if you want to broadcast your Google Voice Number.
You can dot that by;

"Settings > Calls > Caller ID (out)" and select "Display my Google Voice number".

Google Voice Blog: Helping you broadcast your Google Voice number: "'Display my Google Voice number'"