Despite spell check’s proclamations otherwise, readback is a compound word that exists.

Wiktionary defines readback as follows: (1) (Computing) The act of reading data back from a location where it has been stored. (2) (Aviation, telecommunications) The repetition of a message one has received, in order to acknowledge it. (3) The reading aloud by a court reporter or stenographer of testimony previously taken down in stenographic dictation, usually at the request of the presiding judge or parties involved in a deposition.”