Hi There,
Are you referring to retrieving voicemail messages from the phone?
You can try 1234, 12345, or 123456 (common default PINs).
If the PIN can not be guessed, you will need to get in contact with whoever manages the voicemail system. If the voicemail system you are using is Cisco Unity Connection it is easy to reset voicemail PINs to a known value.
Here are the PIN reset steps for Cisco Unity Connection.
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unity-connection-version-80/113327-change-unity-pin-connection.html