Symbol 5211-1985 from IEC 60417 - Title/Meaning: Listen Function/description: To indicate a ""listen"" facility.
Additional information: 
Published in: Supplement G, 1985
Own work using: iso.org/obp/graphics/grs/8a2ec5fd-23da-4bc6-a280-c9c0679d746c 200.png
Vectorization:  Mrmw