A voice note from a friend in another time zone is them choosing you while their day is still going.

Time zones make effort visible. Somebody recording a message at four in the afternoon while your day has not started has interrupted their own momentum to talk into a phone with no reply coming. That is a small sacrifice in a day that had other claims on it. Distance thins friendships out, and the ones that survive it do so because somebody keeps paying this kind of cost.

Send one back, badly, without planning what you will say. A rambling two minutes about nothing is worth more than a polished text. And pick a standing time once a week so the friendship does not depend on either of you remembering.