iPhone Voicemail Button Setting

Phone Operator        GSM Dial Code
-----------------     -------------
3(2G)                 *5005*86*91239123#
3(3G)                 *5005*86*63323232# 
CSL/One2Free          *5005*86*988#
HK New World          *5005*86*60331238#
PCCW                  *5005*86*63888892#
Smartone-vodafone     *5005*86*138#
PEOPLES               *5005*86*218#

Other GSM codes