In Sir Gawain and the Green Knight, you could say the call occurs when the Green Knight infiltrates the court and demands fighting. When Gawain takes up the challenge, he is answer that call which extends to not only fighting the knight but also promising to meet him in the future. (The motivation is his honor, as he needs to uphold his honor as a Knight or it will shame him and his king)
The guide/mentor you might say is Bertilak (debatable since he's also the Green Knight but seems like the best option) who is the one who tests Gawain's honor with the hunting events.
You could call the girdle the talisman since it's enchanted and the men of the court wear a green sash from then on.
