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…
There would be people who might pay for not having to bother to write the code (and maintain it, and maybe innovate on it) for this. You never know.
"it's trivial to write" does not mean people won't pay for it. Things are more complex than that.