It looks like Adtran is OK with failing G.168 echo cancellation tests on their higher end NetVanta VoIP gateways.
This is from their latest release notes:
Using the HEAD acoustics test suite, some G.168 echo cancellation test cases fail on the NetVanta 6240
and NetVanta 644. These same tests pass on Total Access 900 Series units. There is no reason to believe
this would affect a customer in the field.
VoIP4EA 