If you were like me, and didn't answer any of the phone calls during your playthrough, then you would have to go back to do it.
Good news is, you can talk to the same customer 5 times.
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The quest I used was "More light, please" in Coralroot Forest (top of map).
When you spawn in, just walk forward, open the door to trigger the phone call. Press the hotkey to answer the phone.
My achievement popped as soon as i picked up the last phone call - which leads me to believe, you do not need to have the full conversation for it to register. Just pick up, pause -> Go home -> Repeat