Cross-Platform Native MMS Mobile App Creation

I'm looking for a skilled mobile app developer to create a native, cross-platform (iOS and Android) mobile app for me. The app's primary function will be to send and receive MMS messages, including text and various file types (specifically images, PDFs, and documents). The app will need to store these messages and files, but there is no requirement for the user to be able to view them within the app.

Key Features:
- Background Operation: The app will need to operate in the background without human intervention, handling the following tasks:
- Receiving SMS from an external API and relaying them to the user's phone
- Receiving replies from the user's phone, processing these messages, and posting them to our API
- Native Development: The app must be developed natively, without reliance on any online services.

This project will require:
- Extensive experience in native mobile app development for both iOS and Android
- Proficiency in handling MMS messages within mobile apps
- Experience in working with external APIs
- Ability to ensure the app runs efficiently in the background

Please ensure to test the app thoroughly before bidding.