Voice box access differs depending on the device you use, as does the signalling for an unanswered voicemail.

Access via Phone

The voicebox can always be called using the corresponding mailbox icon.
In case of an unread message, the phone signals this in two ways:  A red voice mail icon (1) on the right hand side of the phone An LED light on the receiver (2) of the phone. If you press the button next to the red icon, the voicebox will be called.