It's relatively easy to design a usable Web application if you keep in mind the user's need for a consistent mental model of the control set. I've been designing software user interfaces for twenty years and have that habit, which doesn't mean my first-draft interfaces are perfect but with attention to feedback from perspicacious users (thanks Joy!) they at least tend towards perfection.
Designing the dial-in IVR's control set is a different level of the game. It's as though every Web page were plain text with no links, and the keyboard's keys each did... something, but you couldn't tell what until you hit them. The usual terrible solution is to read off a menu of numbers and options and hope the user can remember them long enough to pick one... or that you can convince them to "please listen closely because our menu options have changed!"
We're not doing that. We want to make a usable phone application that requires minimal education of the user. And while the Golden Rule of interface design remains "don't pull the rug out from under the user's mental model", we're discovering that some new rules apply:
1. Throw out functionality. Yep, just do less. You've got twelve keys, but by existing standards in voicemail IVRs, 0 means help, # means done, * means cancel. So, nine options max. Resist the tempation to make keys context-sensitive; without visible cues users need to know that each key reliably does one thing.
2. Give feedback rather than instructions. Make sure pressing the keys can't do anything immediately destructive, then reward the user for experimentng with the interface by letting them know what's happening. New users, at least, will get subtle notificatons telling them what's happening: just a voice saying "pause", "jump back 5 seconds", "skip forward" as you press the key.
3. Use the key positions. 1, 4, and 7 are on the left side of the keypad, and in our culture left means back. So we use them to go back to the beginning of the current item, or to the previous item, or jump back 5 seconds. "Back" can be a metaphor too: pressing 7 lets you record a reply to the current show. The right side has 3 for next item, 6 for play faster, and 9 for forward the show to a friend - all easy to remember, since they're on the "forward" side of the keypad.