Your 4 digit extension will be displayed in the upper right corner of the phone display in the form AAAA-####, where AAAA would be a 2 to 4 character alphabetic code associated with your company name, and #### would be the 4 digit extension.
Phone lines and mailboxes themselves are based on your phone extension. For example if your extension was 3000, then your first line would actually be 30001.
This is important if you are dialing in to collect voicemail from outside the system as the full extension including the line (all 5 digits) must be entered to identify your mailbox.
Your extension may be used as a label next to the line buttons on your phone.
Some phone models may show the complete 5 digit line number. If you see a 5 digit label you do not need to add a digit for the line number.
Some phones may be configured with multiple extensions to provide for mutliple caller ID presence options (one extension could show your main company number to the called party, and another could show your DID). You may have multiple voicemail boxes as well (e.g. 30001, 30002, etc.).
To determine what your extension is if it is not displayed on the phone, simply select a line, and dial *65 and listen to the voice prompt.