International Calling, Top-Up & Payment App

I want to bring three everyday telecom needs into a single mobile experience. The heart of the project is a robust international calling module, and I need it to excel at the essentials: • High call quality • Low rates • Family plans Alongside the calling feature, the same app must also handle: • Top-up services for prepaid numbers worldwide • Secure payment processing (wallet balance, card support, multi-currency) • A lightweight messaging thread so users can chat while managing their balance or sharing call credits Cross-platform delivery (iOS and Android) with a shared codebase is preferred; if you see a better route, let me know. Please use proven VoIP and payment APIs, keep the architecture scalable, and make sure the UX feels seamless when users move from calling to topping up or paying. Final hand-off should include source code, build and deployment instructions, and a short run-through showing the international call flow, a successful top-up, and an in-app payment. If all three functions work smoothly under normal network conditions, the job is done.