The 3300 will always try to negotiate at the codec it is given in the initial call setup.
If the device or route has been setup in a zone that is compressed (G729) it will stay compressed, if not it will always default to G711.
So you could have a situation like this
Calling device supports G729, trunk supports G729 however the called deice does not, in this scenario the call will default to G711.
Or you could have a device which supports G729 as does the trunk and the called device but the device is not in a compression zone.
Or all of the above but no compression licences on the controller.
It takes time and planning to make sure that G729 is applied.
One scenario that I get regularly is calls forwared to Nupoint voicemail. Nupoint does not support compression so in this situation I have had to route calls over a back to back DPNSS link so that the E2T card is the answering point and thus maintains the compression.
Share what you know - Learn what you don't