PortSIP SDK Integration for Communication App

I'm seeking an experienced app developer to integrate the latest PortSIP SDK into a richly featured communication app for both iOS and Android. This app will connect with my existing Asterisk server through APIs.

Key Features Include:
- User creation
- Real-time balance display
- Call-making functionality
- Fully integrated payment gateway
- Text messaging
- SIP voice calls (Not video calls, just normal SIP calls)

Necessary Skills and Experience:
- Proficient in iOS and Android app development
- Proven experience with PortSIP SDK
- Familiarity with Asterisk and its relevant APIs
- Skills in developing chat features, specifically text messaging and voice calls, within an app
- Experience in implementing a payment gateway in an app

Please note, I have access to and can provide the necessary Asterisk API documentation. Ideally, you are able to show me a similar project you've completed. I'm looking for high-quality work, with clear and frequent communication.