Cause 4 of 4: You're not using your own phone

If you're calling to check your voice messages from another phone, you need to follow a specific procedure.

Solution: 1) Call 021 700 700

2) You'll be asked to enter your "mailbox number".
This is your mobile number without the first 0, followed by # (eg 21 987 654 #)

3) Enter your 4-digit PIN password and press #

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