See attached to provide guidance on cost basis:
https://investors.att.com/stockholder-services/cost-basis-guide/worksheet/atandt-inc-wbd
I am unable to get the cost basis of the AT&T stock since we have had it since 1965 ..including all the splits and various companies... Just know what we have currently. Need to get some basis so I can complete return. Mary V.
There is a very helpful section in the above link from Rick19744 that will help you find the cost basis for the stock you have held since 1965.
In the section marked Effect on Cost Basis is a link to a worksheet for At &T stocks that were acquired prior to March 20, 1988. Follow the instructions about the two-part process.