This is the final version of the Bell logo, which was adopted on October 12, 1969. It was created by Saul Bass and would also be used for Western Electric Company. Bell dissolved in February 13, 1984, leaving behind AT&T. This logo continued to be used by the seven "Baby Bell" companies spun off of AT&T/the Bell System.
AT&T Archives- Saul Bass Pitch Video for Bell System Logo Redesign-2
Saul Bass's pitch film for his Bell System logo redesign