If quality is key, use G.711 unfortunately it takes a lot of bandwidth. G.711 is generally benchmarked with a MOS score of 4.1. But as always MOS is one of those things that just depends. I rarely see G.711 in Service provider deployements of VoIP unless they have “more bandwidth than god”. G.729 has the next highest MOS score as far as ITU codecs go with a MOS of 3.9. G.723.1 (5.3k or 6.3k) both have a MOS of 3.3-3.65 depending where you get your information.
Are you sending both data and voice on the same pipe? If so, LFI in conjunction with various queuing techniques would help prioritize voice traffic.
You can also use RSVP for Call admission control on your links which will help reject calls if bandwidth is gone. If its an H.323 based network use RAI in conjunction with a gatekeeper to signify resource availability which can trigger on processor, bandwidth and available dsp resources. Of course this would depend highly on the vendor’s impelementation of these features.