He shall be called a Nazarene

When did the prophets say “He shall be called a Nazarene”?
The Old Testament mentions neither this prophecy nor even the name of the town “Nazareth,” so “He shall be called a Nazarene” is not a prophecy recorded in the Old Testament.

Then is it an unwritten verbal prophecy from the Old Testament era?
It could be, but after the arrival of the wise men in Jerusalem and before the slaughter of the innocents, when King Herod gathered “all the chief priests and scribes of the people” and “asked them where the Christ was to be born” (see Bethlehem of Judea), their reply made no mention of this prophecy, although it should be noted that they were responding to Herod’s question, which was specific to where Jesus was to be born, not whether or not He shall be called a Nazarene - i.e., Jesus could be and was called a Nazarene because He grew up in Nazareth, not because He was born in Nazareth.

What is the other possibility?
“He shall be called a Nazarene” could be a prophecy issued through New Testament-era prophets some time between Jesus’ birth in Bethlehem and His arrival in Nazareth.

Then why did Nathanael say, “Can anything good come out of Nazareth?” when Philip told him, “We have found Him of whom Moses wrote in the law, and also the prophets: Jesus, the son of Joseph, from Nazareth”?
A prophecy doesn't need to be told to a whole nation. A prophecy can be given to a small group of people, even to just one person, like Mary for example, in which case Nathanael wouldn't have known about it before he met Jesus.

How many “prophets” (Matthew 2:23) spoke this prophecy?
At least two.

Who could be one of them?
Anna the prophetess