Button Triggers for outgoing calls

Helo Guys,

We have a requirement of creating dial plan for outgoing calls initiated from asterisk. Please help me to create a dial plan to implement following requirement,

  1. Tom calls an external number
  2. His call goes to that person’s voicemail
  3. He pushes a button on his phone ,a pre-recorded message is played to that persons voicemail

This is actually not for inbound calls voice mail, but for calls taken place from asterisk to some external numbers. Any help will be appreciated.