An interference waveguide device made of a material with a positive derivative of refractive index over temperature may be combined with a compensating waveguide device. The package has a tray with a central panel for supporting the food product and a flexible flap containing a layer of microwave insulating material for heating the food product. Grooves are formed on the contact surface of the second part so that the respective projections are fitted therein and that have wall surfaces to which the projections can contact at the inner and outer sides of the projections. In this case, the metal wires supply power to the laser light emitting element unit from the transmission side unit via the reception side unit. The invention has a much lower implementation cost than full flash AD conversion, particularly for high-precision conversions. The gateways are VoIP capable, connected to the PSTN and the enterprise network, and mediate between the PSTN fax call and fax over IP calls.