すみません、〇〇線でスマホをなくしてしまったんですが... — Excuse me, I seem to have lost my smartphone on the 〇〇 Line... (Lost · and · Found · Train, Formal — to staff, JLPT N3)

Gesture & etiquette

Put hand to pocket or bag where the item would be, showing distress, but remain polite.

Provide as much detail as possible (train line, car number if remembered, time, specific item description). Do not panic or demand immediate retrieval; lost items take time to process.