How can a customer send themselves to voicemail if they do not want to stay on hold in a queue

How can a customer send themselves to voicemail if they do not want to stay on hold in a queue. ie Customer does not want to keep waiting so rather than hangup they can press keypad combination and be sent to VM. The interrupt promt could explain this BUT what is the keypad combo that allows a customer call coming into send them to VM after waiting a period?