JOEL - I have never heard any church leader nor scripture suggest that the Holy Ghost is just a calling. He is and always has been a member of the Godhead. Some have speculated that if the Holy Ghost is one of Heavenly Father's spirit children, when the duties performed by Him are no longer needed that He will receive a body and continue on in His eternal progression.
Joseph Smith once said that, "The Holy Ghost is yet a spiritual body and is waiting to take to himself a body, as the Savior did."[Joseph Smith, Encyclopedia of Joseph Smith's Teachings, edited by Larry E. Dahl and Donald Q. Cannon (Salt Lake City: Bookcraft, 1997)]
and;
"The Holy Ghost is now in a state of probation which if he should perform in righteousness he may pass through the same or a similar course of things that the Son has." (Joseph Smith, The Words of Joseph Smith, p. 245; Sabbath address, Nauvoo, 27 August 1843. Reported by Franklin D. Richards.)
However even these statements from Joseph should probably be considered as speculation at best, since he nor any other prophet has proclaimed it official church doctrine. We are not even really that certain that the Holy Ghost ever needs to obtain a body. He is already considered a God without one, the same as Jesus was before He came to earth.
President Joseph Fielding Smith was asked the same question and this was his answer:
"There is so much in relation to the gospel that we are required to do and so many commandments to observe and ordinances to receive in order that we may obtain an exaltation in the kingdom of God, that we should have no time to enter into speculation in relation to the Holy Ghost.
Too many members of the Church bother themselves about something which does not concern us and which has not been revealed. They neglect the weightier matters and fail to seek for the light and truth which is revealed and also to obtain and be true to the ordinances and covenants so essential to our salvation."(Answers to Gospel Questions, vol. 2 p. 148)
All we know for sure is what the scriptures tell us:
"The Father has a body of flesh and bones as tangible as man's; the Son also; but the Holy Ghost has not a body of flesh and bones, but is a personage of Spirit. Were it not so, the Holy Ghost could not dwell in us" (D&C 130:22).