It should be on the bottom of the device on a sticker.

Or on the box it came in

Thanks,

First we need to make sure you have recorded an announcement for the Voicemail to work.

To do this, do the following please

Using the telephone base:

Press •/REC while the telephone base is not in use.
Press /CID or /DIR to select Announcement, then
press the SELECT softkey.
Press /CID or /DIR to select Mailbox 1 or Mailbox 2,
then press the SELECT softkey. The corresponding
icon, 1 or 2, flashes on the screen.
The telephone base announces, “Record after the tone.
Press 5 when you are done.” After the tone, speak
towards MIC (microphone) of the telephone base.
Press 5 or the STOP softkey when finished recording.
The answering system automatically plays back the newly recorded
announcement. Press 5 to stop the playback at any time.

Then make sure it is switched on by doing the following

To turn the answering system on or off :
Using the telephone base:

Press /ON/OFF LINE 1 and/or /ON/OFF LINE 2 to turn on the
corresponding answering system. The telephone base announces and
shows, “Calls will be answered.” The /ON/OFF LINE 1 light and/or
/ON/OFF LINE 2 light will be on respectively.
Press /ON/OFF LINE 1 and/or /ON/OFF LINE 2 to turn off the
corresponding answering system. The telephone base announces and
shows, “Calls will not be answered.” The /ON/OFF LINE 1 light and/or
/ON/OFF LINE 2 light will be off respectively

When someone leaves a message, to retrieve it you need to do the following

To play messages using the telephone base:

Press /MAILBOX LINE 1 for line 1 or /MAILBOX LINE 2 for line 2
to listen to messages.
Options during playback:
Press /VOL/ to adjust the speaker volume.
Press /SKIP to skip to the next message.
Press /REPEAT to repeat the message currently playing. Press
/REPEAT twice to listen to the previous message.
Press X/DELETE to delete the current message. The system advances
to the next message.
Press /MAILBOX LINE 1 for line 1 or /MAILBOX LINE 2 for
line 2 to stop listening to messages.
Press CANCEL to exit to the previous menu.

If you have any difficulties, please tell me exactly where on my steps you had the difficulties.

Thank you