This is a major cause of dropouts. Ensure that you do not have any T200 phones
 connected to the same line that the modem uses.

The T200 phone is the standard slimline phone that Telstra issued up to about
two years ago. If your phone is more than two years old, it probably is a T200.
You can check by the model number on the back of the phone. Look for the
number "200" in the model number.

These phones obtain power for their memory from the phone line every
5-20 minutes. When this happens, the line voltage may drop slightly. This is
sometimes enough for some modems to think that the other end has hung up.

Solution:

• Unplug the phone to test it
• Replace the Phone with a new one if possible
• Usually Telstra will swap your old Telstra phone for a new one of theirs if
  your phone has a problem, and since the T200 does I'm sure they will swap
  it if you explain it to them.