Hi bigbossa!
and greetings to a fellow Linuxer
I also have a Billion 7800N and a long (ADSL1) line which (sometimes) is subject to much interference.
One thing that I would say, and you may have already discovered for yourself, is that Broadcoms PhyR feature is not enabled by default.
So if you update the firmware, or reset to default settings, it will no longer be enabled, even if it had been previously.
You can enable it by setting the 'PhyR' check boxes in the modems web interface under Advanced / Configuration / ADSL Mode.
Don't forget to then 'save' the settings so that it is still enabled next time you reboot the
router!
A brief spell with PhyR disabled saw my sync rate and SNR margin hammered by DLM.
Best of luck!