ringback

ring +‎ back

ringback (uncountable)

1. (telecommunications) The sound transmitted back to a caller to indicate that the telephone they are calling is ringing.
2. (telecommunications) A telephone feature allowing a person to receive an automatic call back when a busy line becomes free.
   • 2013, Mark Roberts, The Sixth Soul

     'But you've been engaged an awfully long time: I had to use ringback in the end and that's expensive, but this is important so never mind.'

• king crab