Here’s a website dedicated to that purpose via a petition

If Turkey aspires to join the community of Europe, the first step would be to show a spirit of understanding and a sense of justice, mutual respect and willingness to cooperate, as is the culture of the European Union. The time to restore the spirituality of Hagia Sophia as a place of Christian worship is now. The spiritual “redemption” of the Holy Wisdom church might be realized with the same care and respect with which Abdülmecid, that progressive, proeuropean reformer of the Ottoman Empire, commissioned the last major Ottoman restoration of Hagia Sophia.

History might neglect but it never forgets. Historic mistakes can be reversed and corrected. By taking the big and noble step of redeeming Hagia Sophia, the Turkish state would prove its genuine respect for the Christians of Europe and the world. Restoring Hagia Sophia to Christian worship would be THE revolutionary gesture of wisdom, progress and civility. Such an act would offer Istanbul the rare privilege of hosting not only a great monument of world art, but also one of Christianity’s most important places of worship, a truly rare diamond.