Some modems are just not compatible, and it does not matter what you do they may not work properly, if you can borrow a modem from a friend or someone else to try, you may find that your modem is the problem, we have had a couple of people where they can get a connection but cannot go anywhere, also others the can connect on there fax line and not on the modem line, by changing one or the other or getting Telstra to look at the line even reporting the line a second or third time can also help remedy the problem. If you are connecting to our 56k modems and having problems try calling into the 33.6k lines, the link between the telephone exchange and our equipment is optical fibre all the way. Digital lines don't suffer from noise problems on our end, but the wire between you and the telephone exchange is still copper, also 56k modems are more aggressive with speed negotiation than the old analogue modem. Anything other than your modem connected to the phone line eg, answering machines, other phones, fax machines and even cordless phones call all cause interference to the phone line in some way. Make sure that you do not have your mobile phones, radio transmitters or any other device capable of transmitting and causing interference near your computer or modem.
Modems can be incompatible and no modem is completely compatible with every other modem.
Check out the modem manufacturer for upgrades of either firmware or drivers. Even new modems can have outdated drivers or firmware. Check any extension cable for defects. Ensure that the telephone line is plugged into the line socket and not the phone socket on the back of your modem if it has two outlets that can take a telephone plug. If you find that after you send and recieve your mail you are being disconnected, it is most likely due to an incorrect setting.