DID pointing to Voxalot and Call Quality

[pmerrill]

I've currently got my DID pointing to my voxalot account xxxxxx@voxalot.com and I've registered my ATA with voxalot as well. I have two VSP providers set up, SIPME and ASTRATEL. My DID was provided by SIPME but they have "redirected" it to my Voxalot account as requested.

For incoming calls using my DID, who actually gets involved with the call and how do the codecs get chosen? What would prevent the call being connected using my preferred codec, g711a?

It "appears" that when I call using the Sydney sipbroker access number, followed by *010XXXXXX, the call IS connected using g711a and the quality is great. However, when I call home via my DID, it seems to connect via g729 and the quality is not as good.

What EXACTLY is going on here?