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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:
Note: We have been made aware that on some Samsung phones it is not possible to change the default duration (20 seconds) as they do not send the correct commands to the network.