The call to adventure begins with Bilbo Baggins, and the ring he gives to Frodo, before he travels to Rivendell. It plants the seed of knowledge about the ring and it's power into Frodo's life. The actual call to adventure later occurs when Gandelf gives Frodo Baggins the ring, and encourages Frodo to set out on a quest to Mt Doom, where the ring can be destroyed, and hence liberate the world from the power of Sauron. Frodo is initially reluctant to accept this quest, wondering why Gandelf can't simply destroy the ring himself. However, after meeting The Fellowship and his near-death at the hands of the ring-wraiths, he relises his life and the ring are intextricably linked, and he must destroy the ring, if he is to live peaceably.  And he takes up the call.