I love the fact that you solved a specific pain point and can potentially help others, but I'm not sure this is something that would be worth $10/month for most people. This is also a pretty trivial app to build with the twilio/tropo APIs, so it's not an earth shattering product.

Can you set it up to use the same number, but validate accounts with access numbers? That way you only pay for 1 number and could possibly ask for donations on the site for hosting costs (sounds like you could still get away with the free heroku account for your building).