Cause 4 of 4: You're trying to check your voice messages when abroad

The default voicemail number can only be used if you're calling form your home network.

Solution: 1) Call +64 21 700 700

2) You're asked to key in your "mailbox number".
This is your phone number, leaving out the first 0 followed by # (e.g. 21 987 654 #)

3) Key in your 4-digit PIN and key in #

4) You're now in the main menu. Follow the verbal instructions to listen to your voice messages.