Mix into a state alongside a paginate/1 declaration to page a long listing without hand-rolling the bookkeeping:

defmodule MyApp.States.Products do
  use Ussd.State
  use Ussd.Pagination, per_page: 5

  alias Ussd.Decisions.Equal

  paginate next: Equal.new("#"), previous: Equal.new("0")
  transition Ussd.Decisions.Fallback.new(), to: MyApp.States.ProductDetail

  @impl Ussd.Pagination
  def get_items(_context), do: Catalog.list_products()

  @impl Ussd.State
  def render(context) do
    Ussd.Menu.build()
    |> Ussd.Menu.listing(get_items(context), page: current_page(context))
    |> Ussd.Menu.line(if has_next_page?(context), do: "#. More")
  end
end

Requires get_items/1; per_page can be given either as the :per_page option to use Ussd.Pagination or by implementing per_page/1 yourself.