But when the shepherd calls, his sheep will all come forward and follow him. Not one of his sheep will stay in the fold. The same thing happens as each shepherd steps out and calls his sheep. The sheep will only follow their shepherd's voice. They will not respond to another shepherd's voice.
Someone can try to mimic the shepherd's voice but the sheep know the difference.
Someone can dress up to look like the shepherd but the sheep know by his voice that it is not their shepherd. If we obey Jesus and follow His words, then we are His sheep. We will only follow and obey Him. If someone comes along and calls us to follow them we will not do it. We will know that they are not speaking with the voice of Jesus.