We hear God's voice best when we are in conversation with God. We don't exactly know how God's call "came" to Jeremiah. Perhaps it was in a dream. Perhaps it was an inner voice or maybe even it was during a time of prayer. But, like Moses and Samuel before him, Jeremiah decides to test the voice by entering into conversation with it, even pushing back against it.
God told Jeremiah that he had been appointed to a prophetic mission before he was even born. Having been schooled in the story of Moses, Jeremiah raises a conversational objection. "Ah, Lord God! Truly I do not know how to speak, for I am only a boy!"
God knows Jeremiah, but he also knows himself and what being "a prophet to the nations" will entail. By pushing back in conversation, Jeremiah can sort out whether his call is something coming from within himself, which could be easily dismissed, or whether this was coming from God -- a call that could not be discounted quite so fast.
It's an interesting pattern in the Scriptures -- those who are the most powerfully used by God are those who take the time to test God's call with a conversation. In fact, conversation is one of the keys to a lasting relationship.