Why not "Wem sprechen Sie?"
Or could this also be "Wer spricht damit?", instead of "Wer spricht mit ihnen?" Would one of these be more common (or encouraged) in this instance?
Wer spricht damit? means "Who speaks with it?" and not "...with them?".
Thanks. Time for a long sigh, followed by a review of the “da” words as to what “da” can replace... I had had it in my head that “da” was a catch-all for any third-person pronoun.
da(r)- is mostly for das, sometimes for es. I don't think it can be used with any other third-person pronoun.