Tip1: You can interrupt a voicemail greeting and go directly to leaving your message
Verizon or Cingular: press *
Sprint: press 1
T-Mobile: press #The catch is you should know the network on which the person you are calling is on.
Tip2: You can check the usage/number of free minutes by
Verizon: dial #646
T-Mobile: dial #646#
Cingular: dial *646#
Sprint: *4
Thanks to Lifehacker post