The SkylinkSDK for Android Public Beta defaults to Opus audio(60kbit/s to 510kbit/s) and allows the developer to switch to between Opus and ISAC.
SkylinkJS (Web SDK) uses Opus by default and auto-negotiates other available codecs as needed.
SkylinkSDK for iOS Public Beta 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-