If a cell phone were made with a megabyte or so of memory for digital audio, you could, among other things:
1) Allow your listener to listen to music while you have him/her on hold,
2) Easily record parts of your conversation for later retrieval (for use as "evidence," or a dictated shopping
3) Generate fake "background" sounds with convincing fidelity and volume level to be mixed with your voice, for use in creating more convincing excuse calls (see Pocket Excuse Generator),
4) Download voicemail for later retrieval from the phone when you are outside a service area, or want to replay multiple times without consuming airtime ("Agent Ryan, once you locate the bomb on the sub's torpedo tubes, here's how you disarm it: Step 1...")
5) Generate in advance an emergency message to be sent if you need help in a situation where you cannot speak, such as an allergic reaction to a bee sting near the mouth, or a situation where your voice would disclose your hiding place, etc.
6) Download an audio file of your own choosing to use as your "signature" ring sound. Not a tune to be played on the phone's beep tones, but a recorded bell or fully-orchestrated music, or a car horn, or your own voice, etc. (This application gleaned from Johan's annotation of rmutt's "Real Chime" idea).
7) Make your phone emit a personalized signal at the sound of your voice, to help you find it when it's lost -- suggested by rayfo (see "Personal Ringer" link)
8) Other suggestions...?