Sorry, only Telstra can help you by connecting you with a better line, or fix problems with the existing one. Talk to Telstra if you hear noises during a voice session, i.e., when you use your telephone line normally. Or ask Telstra to give you a data grade or fax grade telephone line. Another way to get around a bad telephone line is to use a lower speed. If you are using a 28,800bps modem, set you connection speed to 9600bps and try again.
Some modems just fail to maintain a reliable connection.
Try a different modem to the one you are using and see the effect. If it works you may need to change your modem to a better and more reliable one. You have other telephone devices connected to the same line. If you are sharing the same telephone line with other devices, be careful. Some telephone devices, like normal handsets and answering machines, are known to cause problems with your Internet sessions.