People find fire-and-forget so useful that they'll put up with stupid, cumbersome text entry interfaces just to get it. I think it happens fairly often that people send text messages in spite of them being text, not because of it.
What if you could fire a voice message? Hold down the Record button on your phone, say "I'm standing in line at the theater now. How many tickets should i get?" and press Send. Your friend's phone beeps instead of ringing, and when they press Play they hear your 5-second message. It's much faster and easier for you than typing in a text message, and much faster and easier for your friend than going through the (still stone-age) process of picking up voicemail. And your friend should have the option of replying by texting you "4" or saying "four tickets please — stay outside and we'll meet you there in ten minutes."
This should be trivial to implement in software on today's phones. These phones can already send pictures to each other; sending sounds would be a piece of cake. The question is, why don't they already do this? Surely this is not a new idea. Everyone knows that the United States is years behind in mobile phone technology, but even more puzzling, why don't phones already do this in Europe or Japan? Maybe they do?