Call pick up is not working in pstn phones

Call pickup is allows to answer an incoming call from another phone.

For Eg: I have three PSTN phones .If i made a call from one pstn to another pstn i can able to pick up the call from the third pstn phone by pressing *9(extension of second pstn) .But here what’s the problem is once i press *9 it is automatically goin to voice mail.can any one please tell me any solution for this problem.

Not without details of your dialplan. If you didn’t write the dialplan, you should contact the person or organisation that did.

i have fallowing dial plans:
exten => _*9.,1,SET(GLOBAL(PICKUPMARK)=${EXTEN:2})
exten => _*9.,n,Pickup(${EXTEN:2}@PICKUPMARK)