Depending on what type of wireless you are talking about voip over wireless can work quite well. Two things you must consider when implementing voip.
Delay (ping time) and Continuous throughput. A high ping time is not going to cause poor voice quality alone, it will create a delay from when you speak to when the person on the other end hears you.
Not having enough throughput capacity is going to cause you to have bad voice quality as the RTP stream is UDP and has no error checking. If your connection is bogged down, or the service is not consistant the voice quality is going to suffer because the packets will just be dropped.
256k Up and Down should be enough to handle 8 simultaneous voice calls without a problem as long as the connection is persistent.
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