Extending The Ring Time Before Calls Are Diverted To Voicemail:
When activated, your mobile phone is set to ring for 10 seconds before being diverted to your Voicemail service. If this is too short or too long, you can change the time in intervals of 5 seconds, anywhere from 5 - 30 seconds.
Telstra 4G : **61*141**xx#
Simply replace the xx with a multiple of 5. For example: **61*321**30# will make the mobile phone ring for 30 seconds before it diverts the call to Voicemail. 30 Seconds is the maximum ring time on all networks.