I'v had a few people ask me about this.
Their iPhone displays 'Telecom NZ' when when on Vodafone.

What has happen is you have put a XT Sim card in your iPhone and when you put your Vodafone sim back in it seems to think that your still on Telecom, when your really not.

How to fix:

1. Turn your iPhone off 


2. Put the XT Sim card in the iPhone


3. Turn on iPhone


4. Go in to Settings --> Carrier


5. And select 'vodafone NZ'


6. Turn off iPhone


7. Put Vodafone sim card in iPhone


8. Turn on


9. ?????


10. PROFIT

I hope this helps as a guide. YOU DO NOT NEED TO RESTORE YOUR iPHONE DESPITE WHAT VODAFONE TELLS YOU!