If there's no one else present, and nothing happens to interrupt the dialogue, then technically there's no need to keep using (into phone)
which is another industry standard or V.O. over and over.
Fun begins when there is someone else present, say a third character, and they start talking, so you have to use (into phone) again to establish who's saying what to whom, e.g. and just for fun's sake (watch the extensions):
INT. BEDROOM - DAY
Mike lies on the bed dozing. Phone RINGS, Mike picks up.
MIKE (into phone)
Hello?
LOU (V.O.)
Hey Mikey, old pal, old chum.
Surprise! Guess who?
Mike sits up fast -- this is the last guy on the planet he wants to hear from.
MIKE
What the hell do you want?
LOU
Don't play innocent, Mikey.
You don't bring the money to
the club at ten tonight,
something real bad is gonna
happen. Don't make me have
to spell it out. You know I
got problems with my spelling.
Maggie enters, checks out her dress in the mirror.
MAGGIE
Who is it, honey?
MIKE
It's, eh, Fred. Asking if
I want to play golf on Saturday
morning.
MAGGIE
Well sure. I'm having my hair
done anyway.
LOU (V.O.)
That sounds real nice. You and
me tee off at ten o'clock tonight.
Be there.
MIKE (into phone)
Now listen, you can't just--
CLICK. Dead line.
MAGGIE
Tell Fred I'm asking for him
and Dolores.
MIKE (into phone)
Maggie's asking for you both.
(to Maggie)
Right back at you.
MAGGIE
How's my new dress look?
MIKE
Good enough to eat.
Maggie grins and exits. Mike slams the receiver down.
MIKE
New dress?!
...all of which is pretty much common sense, all you're doing is instructing the actor when to pretend to talk into an instrument.
Above, I could have put (into phone) beneath MIKE, e.g.
MIKE
(into phone)
Hello?
but I prefer to use MIKE (into phone) -- saves me a line every time.
As has been intimated, there are alternatives to V.O. -- the incoming voice on the other end of the phone.
INTERCUT cuts from person to person in both their locations. Slightly more interesting than one person and V.O. especially if it's a long(er) conversation.
SPLIT SCREEN puts two (or more) people on the screen at the same time. Ideal for romcoms where the leads are having a late night bedroom Doris Day/Rock Hudson type conversation, because we can see both their expressions and reactions.
From all that's been said, here and in the other thread, you are now empowered to figure anything else related to phone dialogue out for yourself.
Read more scripts, secret of the universe.
-Derek
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The practice of art isn't to make a living. It's to make your soul grow. ~The Complete Handbook of Novel Writing (Kurt Vonnegut)