Isaiah Volunteers to Be God's Mouthpiece

Today I read 2 Nephi 16 where Isaiah receives his call as a prophet.  As I was reading this chapter, I got to thinking, if Priesthood callings and ordinations are not sought after, how do we justify that with Isaiah volunteering to be the Lord's mouthpiece?  I understand that the Savior volunteered because Heavenly Father asked for a volunteer, perhaps this is the same idea.  Isaiah didn't exactly seek after this position, this calling, but rather responded to Heavenly Father when He requested someone.  So even though Isaiah did volunteer, it was only after Heavenly Father sought for someone to do His bidding.  In a sense, it would be like Heavenly Father coming to us and asking us if we are willing to do something for Him.  After He asks us, we can accept and it is not seeking after the calling, but rather accepting the calling God intends for us to have.  Until tomorrow.