For those who may be new to the subject, let me take you back to the days when AFOL had dialup units in call bureaus. These dialup units were specifically used on TKL ISDN lines. ISDN standard for voice has been the benchmark for a lot of voip standards. You may google these to get a better idea. In reality, G703/G711 is the only acceptable voice quailty standard however the need for clean internet bandwidth is of utmost importance. ISDN BRI was considered the true mulit-media platform but costs made its acceptance slow. In Voip, your aim is to try and be close to the standard therefore ensure that your network does not suffer from basics like packet drop, jitter. You could use other encoders to compress but the voice quailty cannot be same. Keep this in mind when designing such services or platforms.