The idea of pairing mobile and water needs is really intriguing. On one hand, the prevalence of phones and habits of renewing subscriptions definitely points to an analogous solution. I wonder whether the timing and habits are similar enough (e.g. people don't need to refill their phone credits each day but presumably do need fresh water more frequently than they would mobile). Are there ways to better connect the two markets and people who play a role in both?
You show a solid understanding of who you're designing for, what it means to be sustainable, and a belief in the power of people using their own assets and problem solving capabilities. I'm excited to watch this move forward. One question I'm thinking about is how might you help spread the word about the solution? How can you leverage SMS or other community meet-ups to share and make the solution viral?