Shulchan Arukh, Orach Chayim 256/שולחן ערוך·אורח חיים רנ״ו

1 The Six Shofar Blasts that Would Be Sounded on Friday: When the Jewish people were settled in their land, they used to sound six shofar blasts on Erev Shabbat, [to alert] the people to conclude [their] work. RAMA: At present, the custom in holy communities is that a half hour or an hour before Shabbos, an emissary of the community announces [that everyone] should prepare himself for Shabbat, and this is in place of the shofar blasts that took place in those days. It is appropriate to act thus in all places.