Android Mobile Dialer with SIP

I have an existing mobile dialer application that utilizes softswitch. I am seeking a skilled professional to enhance its functionality.

- I require the application to:
- Support SIP registration
- Make calls using the SIM mobile network rather than the server.

The application should operate similarly to a GSM termination call gateway, where the call is routed through the SIM card on the mobile device.

Ideal candidates for this project should have prior experience in:
- Android mobile application development
- Telecommunication applications
- SIP protocol implementation

The application should be compatible with all versions of Android.