Build SIP/VoIP client for Android

Hello Freelancers, I need a SIP/VoIP client for Android. There are many solutions in the Play Store like Zoiper. I need a simple application with a simple and optimized code. I am familiar with Android coding, so the quality of code will be the main factor in this business. So, please don't bid on this project if you're beginner. The application is just one screen. This screen is like any screen where you enter phone numbers. And then you can perform calls to landlines, mobiles, and SIP endpoints.