Your voicemail comes into play in three situations:
when you're unavailable
when you don't have signal
when you're on the phone
To change whether your voicemail is on follow these steps:
Unavailable / Unanswered calls
Turn off voicemail for unanswered calls: #61#
Turn on voicemail for unanswered calls: *61#
Change the length of time before your call goes to voicemail: **61*121*11*XX# (change the XX to the number of seconds between 5-30 in 5 second intervals)
When you don't have signal
Turn off voicemail for calls when you don't have signal: #62#
Turn on voicemail for calls when you don't have signal: *62#
When you're engaged
Turn off voicemail when you're engaged: #67#
Turn on voicemail when you're engaged: *67#