Does the divorce uttered by the father to the mother over the phone, in the absence of their son, due to his passing by the father’s family without greeting them, take effect? Does it require expiation? And can the son perform it on behalf of the father?

If the father swore an oath of divorce, making the mother's divorce contingent on the son holding the phone, then if the son held the phone in the sense intended by the father, the divorce has occurred according to the majority of scholars, whether the father intended to effect the divorce or not. A contingent divorce cannot be revoked according to them. As for Ibn Taymiyyah, he holds that the divorce does not occur unless the husband intended to prevent something. It is necessary to refer back to the father to ascertain what he intended by his oath, for without that, it is not possible to conclusively determine whether the divorce has occurred. It is advisable to be cautious about using divorce to solve problems.