If your Message Waiting Indicator (MWI) light is on, or you hear a stutter dial tone when you place a call, then you have a message waiting in your voicemail. You can press the VOICEMAIL button or dial 97 to access your voicemail. You will need to enter your voicemail PIN. If you in the office but away from your desk, you can access your voicemail by dialing 98 from another phone and then entering your extension, and then your PIN.
If you are out of the office, you may or may not be able to access your voicemail, depending on how your system is set up. Consult your system manager for details on how to do this.