In order to do this you need to use Repeated Individual Diversion on the Extn.
PELPI:DIR=XXXX,LIST=1,CHO=1,ANSPOS=XXXX,TIME=YY,BSYCHO=2;
PELPI:DIR=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...