Case C-593/19: Judgment of the Court (Fifth Chamber) of 15 April 2021 (request for a preliminary ruling from the Bundesfinanzgericht (Außenstelle Graz) — Austria) — SK Telecom Co. Ltd v Finanzamt Graz-Stadt (Reference for a preliminary ruling — Value added tax (VAT) — Directive 2006/112/EC — Determination of the place of supply of telecommunications services — Roaming of third-country nationals on mobile communications networks within the European Union — Point (b) of the first paragraph of Article 59a — Option for Member States to transfer the place of supply of telecommunications services to their territory)