Is my marriage to a girl permissible if I asked her to have phone sex, knowing that anal sex is forbidden?

It is incumbent upon both of you to repent to Allah, regret what has occurred, and resolve not to return to such acts. We must also draw attention to certain misconceptions mentioned in the question: Firstly, a man approaching his wife anally is forbidden, and the one who does so is cursed, but it does not render the wife unlawful for her husband. Secondly, a woman inserting her finger into her anus, whether incited by her husband or a stranger, does not render her unlawful to the one who incited her. Thirdly, obscene talk over the phone is not the same as direct indecency; each has its own ruling in Islamic law. Therefore, there is no impediment for you to proceed and marry her. Be keen on her being a woman of religion and good character.