Symbol 5501 from IEC 60417 - Title/Meaning: Call-back Function/description: To identify the control by which a call will automatically be made again as soon as the line is free.
Additional information: 
See ITU-T Recommendation E.121
Published in: Supplement J, 1990
Own work using: iso.org/obp/graphics/grs/0ef82745-2267-44a2-ba3d-dae387a58111 200.png
Vectorization:  Mrmw