SkylinkJS (Web SDK) uses Opus by default and auto-negotiates other available codecs as needed.
SkylinkSDK for Android defaults to Opus audio(60kbit/s to 510kbit/s) and allows the developer to switch to between Opus and ISAC.
SkylinkSDK for iOS uses Opus audio by default and allows the SDK user to choose between Opus and iLBC. It also auto-negotiates other available codecs as needed.
You can see how the bitrate is affected by the network conditions here : http://support.temasys.com.sg/solution/articles/5000577818-how-is-the-call-affected-when-the-network-conditions-change-