I have created a package for sip integration in react native app, named react-native-sip-smooth. I built it using Linphone SDK. It is so simple and easy, have written every single detail in the README.md file. Just install and use it in your mobile app. Do check it, this is gonna solve your problem.
Thank you
npmjs.com/package/react-native-sip-smooth
github.com/Ammar-Abid92/react-native-sip-smooth