This would require a modification to the Call Forwarding setting for No Answer. Since TracFone and Net10 do not support user-selectable call forwarding, they have those options removed from their phones, and they seem to block the standard GSM command code from working.
The standard GSM command, in case you are lucky and it works for you is:
* 61 * <VM MAILBOX NUMBER> ** <TIME IN SECONDS> # (SEND)
The VM mailbox number varies by market, but you can typically find the correct number for your area somewhere under the Messages settings menu of your phone. The time (seconds before the voicemail picks up) has to be entered in increments of 5 up to 30 seconds. This command is entered from the Home Screen as if you were going to call somebody, not from the code imput mode in the Prepaid menu.
An example would be:
* 61 * 7879559622 ** 25 # (SEND)
The above example sets the system to ring for 25 seconds before forwarding the call to (787) 955-9622, which is Net10's (actually AT&T's) voicemail number for Puerto Rico. Replace the VM number with the one for your area, and the wait time with 5, 10, 15, 20, 25 or 30, depending on how long you want the phone to ring before sending the call to voicemail.
If the code works for you, wonderful. It works for me on an AT&T or T-Mobile phone, but not on a TracFone or Net10 phone.