Anthony Joshua just threatened Deontay Wilder ... and it all went down during a FaceTime call with 50 Cent

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. 

We KNOW 50 wants to see this fight. Make it happen.