Medi,
Here is the deal on voice quality. You need to measure a couple of things. First of all when you run a compression algorithm, you need to make sure that you have the processing capabilities to handle to it. For instance, if you are going through a cisco router, do a show proc cpu to see your cpu utlization. But typically the MOS will be about 3.3. Keep in mind if bandwidth is available , do not compress if you do not have to. Once again, voip rides on rtp which rides on udp which rides on ip. So figure between 20-28k of overhead. In terms of QOS, what type of ciruit are you going over? You may want to interleave and fragment packets depending upon what type of application you are running over the lines.
voip-guru