This is the least hard problem in startup-dom, because Twilio lets you solve Serious Problems for Serious Businesses and they are ecstatic to pay money for them. (P.S. In case it isn't obvious, don't charge based on what you're paying for Twilio. Underlying phone service: cheap. Application logic: expensive.)

There are a lot of one-week Twilio applications worth 5 or 6 figures to the right people.

A) Find those businesses

B) Sell it to them

I suspect B) is the most difficult, especially for programmers who may not have a deep network into fortune 500s.

I also have a feeling that if you could sketch out a way to do B) in a blog post you may write the most valuable post ever made.

There are lots of "simple and obvious" things that aren't. "Hey, this mold kills bacteria." Sometimes, realizing what's simple and obvious is the valuable bit.

There are tons of ideas out there in the mobile voice arena.

Maybe I could just put a side note on the page saying how much the average meeting costs and asking for donations there?

If it's easy to write the code, you will (or might...) quickly have competitors, that's all. But since you couldn't find any, that might not be the case...

I was thinking of just putting it out for free until it got too expensive, then asking for donations and if that didn't make ends meet putting some restriction on such as advertisements or a time limit or something.