OK first thing to check is that you have a ADSL Line filter on all other phones.
If you already have that try disconnecting the other phones and see if the problem keeps happening.
If it stops when no other phones are plugged in then you may have a faulty filter and the signal is dropping out.
What did the *shudder* telstra tech support say?
Also whats the make and modle of your ADSL modem?
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