Log In to your account and go to the Voicemail page on the right hand side of the screen (8th item down).
Set Voicemail to "Active", set ringing duration, and also your PIN number to retiieve the voice mail.
Ringing duration must be set shorter than the ringing time-out for the incoming call.
Also look at :
How do I access my voicemail? - Voxalot FAQ