The "call" Buck is recieving is the temptations from the ancient wolf time. It is his primordial instincts telling him to join the others. This call effects him in a number of ways. Many times he would leave John Thornton and travel into the forests to try to find the call. Because of this, his only tie to civilization was broken. He left Thornton, and little did he know what would happen while he was gone. The Yeehats killed everyone with John at the time. He then no longer had any ties to civilization, and the call still beckoned him. He then left to follow the instinct of his ancestors and follow the call of the wild.
The call of the wild is Buck's primitive instincts calling out to him to return to his ancestry. It is the main conflict because it confuses Buck to whether he should stay or go.