I've recently figured out that if you have a...
a)2 degrees prepay sim with a phone number and voicemail activated
and
b)a data only device (without phone hardware) eg Samsung Galaxy Camera/Tablet

Callers can not leave messages on the voicemail associated with that sim, when the device is turned on. Until you turn off the device or remove the sim anyone calling you will get a disconnected tone or be told their call can not be connected.

so if you buy a data only device, that can however still receive texts, and use a sim from an existing prepay data account with voice mail - your callers won't be able to leave messages when the device is switched on. 
You have to choose to either be able to receive texts OR have a voicemail service, can't have both at the same time.
what a pity.

The fix is easy, get a proper phone enabled device and buy a separate sim for your data device 
Not sure if this effects tablets. 
notified 2 degrees and suggested they tell their team 
hope it helps someone, took me ages to figure out why I couldn't access my voice mail and why no one could leave messages.