Originally Posted by
grant hutchison
I hear "I'll come back to you" fairly often, and have uttered it myself fairly frequently.
Thinking of context, I say it when I'm in the room with the person I'm talking to, though there's no implication that repeat face-to-face contact is implied--it's understood we're talking about a brief phone call or email. Usually it's in some extended form like, "I'll come back to you on that point," though if then pressed for immediate information I might repeat (holding up a hand in a fending-off gesture), "I'll come back to you."
So "I'll get back to you" when I am distant from my collocutor (on the other end of a phone line, for instance), "I'll come back to you" when we're both in the same location. To me, the difference in usage seems intuitive, but it pivots on the sense of being distant during a phone call; others may feel sufficiently close on the other end of a phone, or even while writing a letter, for "come back" to seem right in that setting, too.