I have a billion BIPAC-7402 not quite sure where to check the logs...
The Billion routers does have this tendancy on certain Dslams.
Go into your router and under your ADSL settings change the following:
Connect Mode: ADSL
Modulation: G.Dmt
And then you might want to play around with: Tx Attenuation
For Tx Attenuation you might want to go check the Whirlpool forums of Australia for guidance.
It seems the Billion Chipset does not always succesfully negotiate the correct settings with certain Telkom Dslams and with these settings you force the correct values.
If you get upgraded to 10meg though you might need to play around with the settings again since ADSL v1 can only go to 8 meg if I remember correctly.