AT&T's stock is doing well this year, but there's still plenty of room for the rally to continue.AT&T (T -0.51%) is a good dividend stock thanks to its high yield. At 5.9%, it's more than four times the S&P 500 average of 1.4%. For income investors, it can make an ideal investment for the long haul. But there's even more reason to invest in AT&T: the potential upside. Shares are up more than 30% in the past 12 months, but it is still a sharply discounted stock. Here are three reasons it could still rise in ...