Don't believe B737 has a pilot response warning or anything similar.
At the end of the route I think you get FMC message and autopilot stays on. Nothing particularly loud that I'm aware of.
Comms and radar coverage spotty in this part of the world. Addis probably wasn't aware they weren't descending until they were tens of miles out.
Cabin crew should be checking though. Maybe Ethiopian don't do that or let it slip.