If you dial the voicemail system number, then press # when it asks for your PIN, you will then be asked for the extension number, of the mailbox, you wish to retrieve messages from.
So...if you have a speed dial key, that allows pauses, and the # key after dialling 999 (or your VM system access number), it could be assigned to a key, otherwise, just press the voicemail key on your set, and complete the rest manually.