How do I extend the number of rings before my mobile diverts to voicemail?

In order to extend the number of rings before your phone diverts to voicemail follow these simple steps:

First, dial *#104# (press send) - this will display your international mailbox number in the format +447xxxxxxxxxx - make a note of this.

Secondly, dial the following: **61*(then your intl mailbox no)+447xxxxxxxxx*10*30# then press send.