In Siddhartha, the call is internal. Something inside Siddhartha (maybe those mangoes he ate for lunch) is telling Siddhartha that his current mode of existence is unacceptable, and that spiritual awakening needs to be found elsewhere, outside of his hometown.
The two friends wander, which is like going on a journey without having a destination.
After he quits the life of a wandering Samana, Siddhartha experiences a rebirth (in the chapter conveniently entitled "Awakening") which carries him closer to his goal of enlightenment. However, his new life of sex and money derails his plans and he loses sight of his original objective. Siddhartha is sucked into the greed, lust, and jealousy of the material world. When he finally walks away from the material life, he experiences intense despair and contemplates suicide.
Although he is well-poised to achieve enlightenment, Siddhartha must listen to the river’s teachings and internalize them. Additionally, he must reconcile his profound love for his son and his belief in life’s transience. These obstacles to enlightenment are the most difficult to overcome.
With the help of Vasudeva, Siddhartha realizes that all voices are one – that everything in the universe is connected. Not only does he realize this, but he deeply internalizes it and thus attains enlightenment.