Your phone should have options to specify how and when calls are diverted to your voicemail.
If you phone does not have menu options for this you can use the following codes to control the diverts.
You dial the following as if making a phone call
Divert All calls to voicemail
Divert on No Answer
Delay nn seconds: max 30 seconds, in 5 second increments
Divert on Unreachable
Divert on Busy