I am no expert on this, but from my understanding:
If you call me, even though you may want to force a call to
MyNumber@au.voxalot.com, because I am using the VoXalot US cluster, the call will be placed to
MyNumber@us.voxalot.com (so from my understanding the cluster used, depends on the cluster in which the person you are calling is registered)....
Martin can probably step in with a more correct explanation....