I changed out mine years ago, You can check with them to see if your stock was tendered, I'm surprised you never received any contact on it. I did not send in one share of GTE stock for Verizon and got letters and calls. I wanted to keep it in my collection, so I told them i must have lost it.
The last two splits for AT&T (T) were on March 20, 1998 (2:1) and May 26, 1993 (2:1), courtesy Ameritrade.
The merger was approved in an annual meeting on April 28, 2006, and presumably happened a couple of months later (I still have the notice of that meeting).
It is easy to get historical stock quotes but harder to get quotes for historical stock symbols, which SBC is.
Based on my own holdings, there were no SBC stock splits between early 2004 and the merger, and the merger turned SBC into T on a
1:1 basis (dividend stubs mention the number of shares I had before and after the merger, and that didn't change). They didn't replace old certificates or issue new/additional ones. A brokerage account would reflect the change, though.
My conclusion is that you bought 200 AT&T shares at some point. The other 400 shares came from the merger.
A quick look at SBC shows it no longer exists, so your hypothesis is almost certainly correct. If a perusal of T's stockholder relations portion of their site doesn't look promising, I'm sure they'd happily answer an email to that department.