How did the Virgin Birth of Jesus prove to be a “sign” as prophesied in Is 7:14?

WE read in Is 7:14 (KJV): > Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. Of course, the Gospels speak of the Virgin Birth of Jesus. But, before the Gospels were recorded, there must have been a way in which the people awaiting the Messiah became aware of the virgin birth of the Son of God. In fact, Joseph must have taken Mary home as his wife, on the initial days of her pregnancy, as we see in Mtt 1:24: > When Joseph woke up, he did what the angel of the Lord had commanded him and took Mary home as his wife. That would mean that Mary, for the public eye, was a married woman staying with her husband before the childbirth. We do not see the Gloria- singing angels or the Wise Men from the East mentioning the virgin birth. On the contrary, the public referred to Jesus as the son of Joseph (Mtt 13: 55) In fact, very few people including Mary and Joseph, a couple of their relatives like Elizabeth and some of the disciples knew of the virgin birth of Jesus by the time he entered public life. But then, whom was the sign as mentioned in Is 7:14 meant for? My question therefore is: **How did the Virgin Birth of Jesus prove to be a “sign” as prophesied in Is 7:14?** Inputs from any denomination are welcome.