Codec Selection - How does it work?

[pmerrill]

I'm trying to figure out how codec selection works from end to end. Here's the situation. My Draytek 2100V is setup for g726 (32 KB). I've also setup my voxalot account and indicated that my provider supports g726 in the codec field.

Now, when I make a call to my work number in Sydney what exactly happens? How do the codecs get chosen at voxalot and at the VSP?

The 2100V has a status page that supposedly reflects what's happening to the call and the codec being used. When I made the call this morning it indicated that g729 was being used even though I specified the 2100V to use g726. Obviously some negotiation was taking place to select the right codec.

So what's exactly happening?