Hello to everyone,
Will I be mistaken if I use one word "callers" to describe "firm's visitors" and "firm's telephone interlocutors"?
It might be a little confusing; I haven't myself heard "callers" used for "visitors to a company". On the whole, "callers" is not much used for corporate visitors; it has a more domestic air.
So I would be inclined to say "callers" for people who telephone and "visitors" for people who visit.