In order to do this you need to use Repeated Individual Diversion on the Extn.
PELPI

IR=XXXX,LIST=1,CHO=1,ANSPOS=XXXX,TIME=YY,BSYCHO=2;
PELPI

IR=XXXX,LIST=1,CHO=2,ANSPOS=ZZZZ,TIME=60,ONECAL=0;
Where: -
XXXX = IP Phone Extn No.
YY = The time in Seconds that you want the phone to ring before diverting.
ZZZZ = The Pilot No of the VMail Hunt Group.
Other things: -
ONECAL=0 means that more than one call is allowed to Vmail at a time from this Extn.
BSYCHO=2 means that if IP Phone is busy calls will divert to VMAIL, if you do not want busy divert Leave this out.
Other interesting fact here is that if the IP Phone Logs Off the calls will divert straight to VMail Automatically.
Repeated Individual Diversion is a Licensed Object but One License is included per IP Extn License.