Lost Item Prevention Phone Number Keychain. I attach these phone number keychains to all my keys: house keys, bicycle keys, and car keys. If I drop them somewhere, the person who finds them can call me, which is very helpful. This is an essential item, especially for keys that children carry. If you need multiple keychains, please let me know the quantity in the comments section. From the second keychain onwards, they are available for 400 yen each.