bill - 7:28 am on May 24, 2013 (gmt 0)
I can't really tell if this is optional or mandatory.
It's an option you have to turn on in your settings.
It had better be optional because they don't support my mobile carrier. ;-p
In my neck of the woods, "multi-factor" technically means it goes both ways
What you're describing sounds like "mutual" authentication.
According to Wikipedia, "Multi-factor authentication (also Two-factor authentication, TFA, T-FA or 2FA) is an approach to authentication which requires the presentation of two or more of the three authentication factors: a knowledge factor ("something the user knows"), a possession factor ("something the user has"), and an inherence factor ("something the user is")."