Went to Vodafone Manukau City, one next to the food court. The lady there was very unhelpful. She said "well since I haven't had the phone with me the whole time, I don't know if you or someone else has changed the pin code" and said it in a very sarcastic and rude tone, basically accusing me of changing the PIN and forgetting it. So she said I had to pay for a new SIM so I just left and went to the one near Number1 shoe. They were way more helpful. Tried the SIM in about 4 different phones, and offered to complain for me about the lady in the other store. They said there wasn't much they could do but recommended I call the help desk and try get a replacement SIM off them.
Called them on the drive home (headset) and they agreed to send me a voucher by text. That arrived and I'm going to go do it after my exam tomorrow morning.
I'm still left wondering though, why would it do this to begin with? The SIM works in other phones, just not that one. And other SIM's work in that phone, but not my phone. One of them said it could be that I've set a PIN so it would only work on my phone, but then after they tried it in other phones they decided against that theory because it would do it for all phones if that were the case.
Will post back tomorrow if anything happens.