Merci à tous pour vos réponses. Comme elles étaient contradictoires, j'ai pris mon courage à deux mains et j'ai écrit un mail à OWO. Voici leur réponse :
Thank you for showing an interest in One World Observatory. As an alternative to presenting printed tickets, our guests also have the option of showing their electronic tickets for entry. Simply access your confirmation email and click on the 'Print Tickets' link to load your e-tickets. Taking a screen shot of each barcode will ensure not having to depend on a WiFi signal to load them once you're entering One World Observatory. If you have any further questions or concerns, please do not hesitate to ask.
Best Regards,
L'utilisation des billlets sur téléphone est donc possible :)