It is the user name / password for your google talk account (for most
people that also tends to be the google voice account).
I don't think you will get notification via hangouts though since that
is not based on XMPP. (ie on a phone you will need to install another
third-party GTalk client)
Yea all of those passwords are the same: Gmail, Google Voice, etc, but the credentials would not take. I am entering my full email of email@example.com and typing in my password. I even log out of Google Voice and then log back in with those same credentials to confirm those are it, but it is still not taking my info in GVMax GTalk setup.