Is it permissible to flirt with a religious girl over the phone, especially since she is a cousin and has currently refused marriage to complete her studies, knowing that the questioner wishes to marry her and communicate with her? And what is the solution if flirting is impermissible?

A man's flirtation with an unfamiliar woman is impermissible, even if he intends to marry her. The lawful approach is to propose to her guardian. A girl should accept a man of religious commitment and good character and not delay marriage by waiting to finish her studies. If you wish to marry the girl, ask your female Mahrams to persuade her to accept, and be patient until the marriage contract is concluded. If she does not desire you, then seek someone else from among those who are religious, and remember the words of Allah Almighty: "And it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know." {Al-Baqarah: 216}, and His words: "And be patient, for indeed, Allah does not allow to be lost the reward of those who do good." {Hud: 115}.