The Yom Kippur observance earns atonement for sins that one commits "Ben Adam La’Makom" – meaning, against only God. When it comes to sins committed against a fellow Jew – "Ben Adam La’habero" – the Yom Kippur observance does not suffice. One must also approach his friend and ask for his forgiveness. This includes cases where one harmed his fellow financially, insulted him verbally, or otherwise wronged him. In all such situations, it is necessary to not only repent, but also to approach the victim before Yom Kippur and ask forgiveness.
The Ribebot Efrayim (Rav Efrayim Greenblat of Memphis, contemporary) addresses the question of whether one fulfills this obligation by requesting forgiveness via telephone. On the one hand, expressing an apology by phone involves direct communication, as opposed to a written apology, but on the other hand, apologizing by phone does not involve the same degree of shame as apologizing in person. A major component of the requirement to apologize is for the perpetrator to feel subdued and ashamed, besides actually earning forgiveness, and this sense of shame is not experienced via telephone to the same extent as through a personal apology. The Ribebot Efrayim rules that one satisfies the requirement of asking forgiveness by apologizing by phone, though it is certainly preferable to approach the victim in person to apologize.
Apologizing by writing a letter, however, does not suffice, because one cannot ascertain that the victim received the letter and granted forgiveness. By the same token, Rabbi Bitan (contemporary) writes (p. 310; listen to audio recording for precise citation) that one does NOT fulfill the requirement by sending an apology by fax, email or SMS. He writes that these media are even less personal than ordinary letter-writing, and thus apologizing in this fashion does not fulfill the obligation of asking forgiveness.
If a person spoke Lashon Ha’ra (negative speech) about his fellow, but his fellow never heard about what was spoken, he should not tell him what he said. In such a case, one should request forgiveness by saying something to the effect of, "Do you forgive me for anything I might have done?" without specifying the content of what was said. If he would inform his fellow of what he said, this would likely ignite tensions between them, which is, of course, the precise opposite of the intent behind the requirement to apologize.
Summary: One is obligated to request forgiveness before Yom Kippur from anyone he had wronged. It is preferable to apologize in person, rather than by telephone, but one fulfills the obligation if he apologizes by telephone. One does not, however, fulfill the obligation if he apologizes through indirect media, such as fax, email or SMS.
