Green’s Dictionary of Slang

(US) the American Telephone & Telegraph Inc., orig. Maw Bell.

1926		Loomis ‘Lineman’s English’ in AS I:12 659: He must beware of all ‘Maw Bell’ (telephone) wires.
1947		Sat. Eve. Post 10 May 16: [Bell] is apparently interested in their every waking and sleeping hour—a maternal solicitude which is largely responsible for the title of Ma Bell [HDAS].
1969		Business Week 6 Dec. 192: Ma Bell’s ‘next best thing to being there’ will become far better than trying to get there [HDAS].
1971		J. Simon Sign of Fool 36: Anything even remotely connected with ‘Ma Bell’ was stamped ‘bell system’.
1980		M. Moskowitz et al. Everybody’s Business: An Almanac 421: Since then Ma Bell has been dragged into a brand new arena: free enterprise.
1997		Lynch & Rothchild Learn to Earn 69: Since that crucial ruling, dozens of other companies have come along to compete with Ma Bell and the Baby Bells.
2003		‘Markets Report’ USA Today 6 Jan. 🌐 With the bulls out in full force in the telecom sector, even AT&T [T] was able to limit losses despite bad news. Ma Bell fell 0.7 percent to $27.48 after announcing a $1.54 billion charge and 3,500 job cuts.