What is the ruling regarding a mother who swore an oath not to buy a phone card for her son, then the father bought it without her knowledge?

If your mother swore not to buy you a phone card, and then your father bought it for you, there is no sin upon your mother, and this is not a breach of her oath. For a breach of an oath occurs when the swearer does what he swore to abandon, or abandons what he swore to do, willingly and remembering his oath.