While you wait for answers, we may have found some information which could help you.
Click here for Handset Cordless PhoneTelstra 7100A, "Not Registered"?
Telstra v850a handset reads line in use. Can't get a line?
telstra 8950a?
How can I add a handset to telstra v580a?
How to fix Telstra 8950 answering machine?
telstra 6100a with 4 handsets?