All a Clone by the Telephone - Tales from the Darkside S1 E11:
Meek struggling screenwriter Leon (Harry Anderson) is contacted through his answering machine by a more confident and successful version of himself from an alternate universe. Making it known how intensely strong-willed he is, the alternate Leon gradually begins to take over the original Leon's life.
Written by Haskell Barkin.
Original air date: January 20, 1985
Talks from the Darkside intro cover by Matt Cannon/Lapses