This setting is between your phone and the network.
To change the setting dial the following code as though you were making a phone call.
**61*+447953222222*11*XX# where XX is the number of seconds before the call is sent to voicemail. This can be any number from 5 to 30 in increments of 5. i.e. 5, 10, 15, 20, 25 or 30.
For example, to increase the time to 30 seconds before the call is sent to voicemail you would enter:
**61*+447953222222*11*30#