For a shared mailbox (called a Shared Extension on the new OneBox system), unlike with the old Octel system, your mailbox number and extension number are different numbers.
If you had a shared mailbox on the old system at extension 3344, you accessed your voicemail with your extension number plus a number for the box - for example, if your mailbox was #1 on that extension, you would access your voicemail by dialing the voicemail number (6999), press the * key, then enter the mailbox number followed by the number 1 (33441) when prompted.
On the new system, your mailbox number is not associated with your extension number, so you might be at extension 3344 but your mailbox number might be 2212. This means that your voicemail box number will never change, regardless of whether you change locations and extension numbers. If you're not sure which type of mailbox you have, contact your Key Advisor or TNS User Support at x5000.