Got hold of a Telstra Monitor 420 answering machine from a garage sale like we did?
It is pretty straightforward to operate, but I really wanted to increase the default number of rings before it answers.
Ingram Micro does the tech support for them on 1300 369 193 (in Australia) and they were good enough to send me a copy of the manual in PDF.
You can download it: Userguide_M420.