Hi Ahmed! Thank you for your question and sorry for the late response (smiley)

In your case I will suggest you to consider two options. First is to create plugin, set up MethodChannel or StreamChannel and add native code for Twillio. I suggest that you start a new Isolate from native code (something like here: https://pub.dev/packages/android_alarm_manager#-readme-tab-) . This is by no means a simple solution. Besides that, you will need to be sure that your project is migrated to AndroidX and that you integrated Twillio SDK correctly and that you can access all Twillio classes and methods. This is the Twillio voice quickstart for Android : https://github.com/twilio/voice-quickstart-android and this is the Flutter plugin for Twillio video: https://gitlab.com/twilio-flutter/programmable-video/-/tree/master/programmable_video which is something similar to the plugin which you need to implement for phone calls.
Second option is to implement this feature in native, which is my recommendation. You will have many less things to think about, you just need to integrate necessary dependencies, also debugging will be much easier.