I have lost divident checks. How do I replace them?

You need to contact the transfer agent for AT&T stock and ask for replacement checks. The transfer agent is a company called Computershare. Contact information is available on AT&T's shareowner services web page: https://www.att.com/gen/investor-relations?pid=278

You may want to register your stock holdings with Computershare in an online account, which will allow you to see the status of dividend payments and easily report any missed checks. You may also want to consider transferring your stock certificates into a brokerage account - that way your dividends would be automatically deposited into your cash account and you no longer run the risk of missing a dividend check.