OPUS vs ISAC on 2G networks for voice calls

[Rahul Pathak]

I have read in quite a few places that OPUS is the best codec for voice calling. But we are facing issues using OPUS in poor/2G networks for voice calls on iOS and Android.

OPUS has a great FEC algorithm, but in poor networks it introduces too much delay. We tested ISAC and found it to perform better than OPUS in those network conditions. We tried OPUS with 8K and FEC: On/Off, but that too did not yield better results. Also the bandwidth consumption was very high compared to ISAC.

Thus, I would like to know-  

1. Is OPUS ideal for 2G/poor networks or is ISAC better?
2. If OPUS is better then what settings do we use for better performance? We are using VoiceEngine directly, hence we can make changes directly in the WebRTC source also if needed.