Play video content EXCLUSIVE Reelz

Charles Manson made several phone calls to a documentary film crew in the final year of his life ... which they're now releasing to the public.

TMZ got this exclusive look at the REELZ project called "Charles Manson: The Final Words."

We're told the doc -- which has been in production for about a year -- was initially scheduled to come out in 2018, but the network decided to bump that up, because as they say ... strike while the iron is hot.

It features phone convos with Manson from Corcoran State Prison -- the most recent one in July. Rob Zombie narrates the documentary, which presents a new theory behind why the murders happened.