One very good reason not to answer the phone is that you don't hear it.
10 seconds sound clip from the Champagne for Caesar movie soundboard.
You can hear this line at 01:07:26 in the DVD version of the movie.
Quote context
[...]
- Would it be so easy for you, Flame, not to see me anymore? It wouldn't be from me I assure you.
- No, it wouldn't be for me either.
- Be here at 8 and don't let's talk about yesterday anymore.
- Oh, it's awfully sweet of you to forgive me and please, don't cry anymore. Good bye.
- Five minutes in the shower? I called for four hours. Women.
- All right, let's forget the shower for the time being.
- One very good reason not to answer the phone is that you don't hear it.
- No, there's no explanation that I can find.
- Except of course that you don't hear the telephone when you're not at home.
(Parrot speaking) I'll buy that.
- Oh, come in darling, come in.
[...]