Telstra Cable uses IP (layer 3) keepalives for the PPPoE session. That means that when you establish an IPSec VPN session with split tunnelling disabled (as most corporate security policies mandate), your PPPoE client can no longer send its IP keepalives to the Telstra server since they now go down the IPSec tunnel. The default timer for these keepalives is 10 seconds, and after three timer sessions have expired without a keepalive being received by the server, the session is dropped. So 30 seconds after connecting the VPN, the PPPoE session dies.

Optus do not use IP-based keepalives. In my experience, IPSec VPN's without split tunnelling will not work on Telstra Cable, while they will work on Optus Cable.

There was a sneaky workaround on some of the older IPSec VPN clients (such as Nortel Contivity 4.8) where the PC's route table could be manipulated after the tunnel is established. But savvy VPN administrators configure their VPN servers to mandate a later client version that will not permit route table manipulation after tunnel establishment.