Around Wall Street, smart people have learned two things in the last decade. One, pray you never get a letter from Jesse Cohn. Two, you ignore Cohn at your own peril. For those of you who have not heard of him, Cohn is the badass Elliott Management activist-hedge-fund manager who has built an impressive record of getting his way with corporate boards that may at first underestimate him or fail to act on his “suggestions” on how to improve the companies they oversee.

He also has been known to help convince boards of directors to defenestrate the C.E.O.’s that fail to heed his requests, which usually appear in the form of lengthy, carefully crafted letters that are based on years of research and often include the idea that the C.E.O. in question should depart. This is a lesson that Randall Stephenson, the longtime chairman and C.E.O. of AT&T, may have to learn the hard way.