It depends on your location. There are various websites that allow you to look up the start and end times of Shabbat based on your location.
Please see the related link for one such site.
Every Friday afternoon at sundown.
The Sabbath begins Friday at Sundown, and ends on Saturday at Sundown. This is true for all practicing Jews, not just Orthodox Jews.
Firstly, in this question there is a misunderstanding of the 'day' of the Jewish Sabbath. Saturday is commonly believed among non-Jews to be the Sabbath. This is incorrect. The Jewish days begin at sundown. The Jewish Sabbath begins at the start of the seventh day of the week. Therefore, the Jewish Sabbath begins Friday at sundown and continues until one hour after sundown on Saturday. If one lives in a city where the sun sets at 3 p.m. on Friday, then the Sabbath begins at 3 p.m. on Friday and continues until 4 p.m. Saturday. If the sun sets at noon on Friday...; etc. Now, to answer the question. Jewish people cannot use a phone (to receive a call or to make a call) on the Sabbath. Once the Sabbath has passed, yes, a Jew can use the phone.
Shabbat (Hebrew for 'Sabbath') begins slightly before sunset on Friday afternoon, and ends after twilight on Saturday night.
The week starts on a Sunday on most North American calendars, since the norm is to begin with the Christian sabbath. (But if you are Jewish, the new week starts on a Saturday, since that is their Sabbath, while for Muslims, it's a Friday.) However, it should be noted that some people regard the start of the week as Monday, the day when most people go to work: in western cultures, the Monday-through-Friday schedule is widely observed in businesses and schools.
Hebrew calendar days begin and end at sunset so the Sabbath begins at sunset Friday and ends at sunset Saturday - includes the night portion of Friday and the daylight portion of Saturday. This follows the Creation account of Genesis 1.
The coming Rosh Hashana will begin at sunset on Wednesday, September 8, 2010, and ends when the Sabbath begins at sunset on Friday, September 10.
The Sabbath occurs every week.
In Judaism, based on Genesis ch.1 and Leviticus 23:32, all days including the Sabbath last from sunset to sunset. In actual practice, we begin our Shabbat a short time before sunset (on Friday) and end it after twilight on Saturday. This is done for several reasons and is a little longer than the actual Biblical day of Sabbath.
Days begin and end from sunset to sunset just as the Creation account in Genesis notes. So the Sabbath began on sunset May 1st and ended sunset May 2nd - our Friday night and Saturday day.
From sunset on Friday evening to sundown Saturday night is considered the Sabbath (Shabbat) for the Jewish people. On Friday at sunset, at least two candles are lit and blessed to welcome in the Shabbat. Traditionally, this is a role of women in Judaism. During Shabbat, there are three meals eaten: One on Friday evening, two on Saturday. At the first two of these meals, challah (the twisted bread) is blessed and eaten. The meals don't begin without two loaves of bread. Any whole loaf of bread can be used, but it is tradional for most Jews to use challah.
God sanctified the Sabbath (Genesis 2:1-4). It begins either through our initiative or by default. Through initiative, if one accepts the Sabbath early, such as when women light the candles before sundown, or the men say the Lecha Dodi prayer before sundown. By default, the Sabbath begins at sundown on Friday in any case, such as if one was sleeping.