asterisk users - May 2009 - Network settings and quality of voice

Hi All;

I discover that there is a relation between voice quality and the network
settings and configuration on the Asterisk machine.

For example, in the sip.conf, if I set the
localnet=xxx.xxx.xxx.xxx/yyy.yyy.yyy.zzz wrong, then this will effect on the
quality of the voice to certain level, actually the voice will be slow (like
slow talking), like robot :)-

I correct the setting and I got a little bit enhance in the voice quality. My
questions is:

1) What kind of settings or configuration related to the network that might
effect on the voice quality?

2) What kind of settings that will help to better resistance to the internet
bandwidth effection (so better voice quality)? I am looking for something that
help the voice packet to be manipulated well even if bandwidth effected?

Any experience that can help in this direction?

Regards
Bilal