To check your own voicemail messages from your telephone, simply press the Envelope key, or dial 1571 on the keypad.
You will then be prompted for your PIN. When you enter the correct PIN, you can then press 1 to enter into the voicemail menu.
To check voicemail for a different number (i.e. the main number) from your phone, press the Envelope key, or dial 1571 on the keypad.
Next, instead of entering your PIN, press the * key.
You will then be prompted to enter 'the extension of the mailbox you wish to enter', followed by the corresponding PIN. Next press 1, to enter into the voicemail menu.
You can also have Voicemail to e-mail configured through Connect so any voicemails will be emailed to you as attachments.
More phone tips and tricks are found on the Polycom user guide.