Joshua -- the reigning WBA, IBF and IBO heavyweight champ -- was chatting with the rapper on his phone when his friend asked 50 for a role on "Power."

That's when AJ chimes in, "I told him we're gonna f*ck up Deontay Wilder! We're comin' soon!"

Deontay -- the reigning WBC heavyweight champion -- has been calling out Joshua for a while ... hoping the two can clash in what would be a pretty epic heavyweight showdown.

Joshua is 19-0 -- and knocked out Wladimir Klitschko earlier this year. Wilder is 38-0 with 37 knockouts.