I'm a 30+ year veteran of AT&T and, somewhere along the line, picked up shares of AT&T & Comcast, the latter probably from a spinoff at some point. The problem is, I have no idea of when I acquired these securities or their cost basis.

I'd now like to sell them.

Is my only recourse to declare all of the proceeds from the sale as a capital gain (i.e. as if the original purchase price was zero)?

Thanks for your help!