Flutter ASTPP Dialer App

I would like a single Flutter code-base that compiles cleanly for both iOS and Android and functions as a soft-phone for my ASTPP switch. The primary workflow is “calling-card” style:

• A new user opens the app, authenticates once via their phone number (SMS or OTP). Then the app “remains logged in always” and doesn’t need login at every reload
• The current prepaid balance, pulled from ASTPP, appears on the home screen.
• From there they can top-up instantly with a credit or debit card (via Stripe).
• After purchase the refreshed balance is shown and the user can place outbound SIP calls through ASTPP.

Key requirements
– Built with Flutter (Dart) so future updates remain unified across stores.
– SIP stack integrated (pjsip, dart-sip-ua, or a comparable library) with reliable background/foreground handling and Apple VoIP push notifications on iOS.
– Clean, minimal UI: dial-pad, balance display, contacts, call & payment history.
– Secure phone-number authentication flow.
– Payment form with card tokenisation; server-side webhook so ASTPP instantly credits the account.
– Source code, build instructions, and basic deployment documentation as the final deliverable, plus a signed APK and TestFlight-ready IPA.

If this fits your skill set with Flutter, SIP, and payment integration, I’m ready to move quickly.