If you don’t need the money and have huge gains, then I think you should probably hold. This is not a commentary on ATT stock value, but a strategy based on current tax law. If you hold the shares until your death — under current law — your heirs will get a “cost basis” of the value on the date of your death. If they then sell the shares immediately, they will pay no capital gains tax.
If, onthe other hand, YOU sell the shares now, you will have to pay gains. And those are tough to calculate! Here’s a link to the ATT shareholder relations section that describes the way to calculate gains on sale of long-held ATT stock.