The paper studies how the optimal nonlinear quantity-payment allocation can be truthfully implemented by optional tariffs in a differentiated goods duopoly. Consumers choose from a menu of tariffs and are subsequently billed according to this. We find that implementation by simple two part tariffs may not be a feasible strategy in a duopoly because the optimal nonlinear tariff exhibits a convexity for lower quantities. We show that the optimal outcome can be implemented if the firms can use two part tariffs with inclusive consumption. The fixed fee includes a free consumption allowance, whereas subsequent consumption is charged according to a steep unit price. That way the firm is able to secure voluntary participation without violating the incentive constraint. The paper show some examples from the telecommunications industry where firms offer two part tariffs with free call minutes to low demand segments.