And then, I called up SBC to get phone service...
Apparently, providing your social security number is voluntary; however, they require your driver's license and date of birth if you don't give it. In my case, even though they managed to come back with my full given name, they said "we still can't identify you; you'll need to fax your driver's license and your social security number to us or we can't give you service."
Note that I previously tried to sign up for service on their webpages; the first time, it got to the end and then bombed; the second time it went all the way through, but they then sent mail saying, "We can't process your order, you'll have to call."
Supposedly, they're going to have my phone service activated in three hours (!!). I'll believe it when I see it.