In regards to the VoIP not working on the VDSL interface, I was told that it wasn't required as VoIP wasn't to be sold with VDSL and therefore disabled. I wasn't able to get anything more specific than that. I'd really like to know if it came down to a licensing issue or if they just "made a choice" to disable it.
Either way, we were fed up with all the issues and general instability of the modem (on both firmwares). An account credit solved the problem by purchasing new reliable hardware.