This was talked about here previously, and it may/may not be a different issue but we had the problem when putting a XT SIM in a Vodafone iPhone, when we put the VF SIM back in it still said Telecom NZ.
and... It was ROAMING on Telecom NZ's network, incurring roaming charges from what others here said.
The fix is to put the XT SIM back in, then choose Vodafone NZ in the network selection, THEN put the VF SIM back in.
The reason you cannot choose the carrier manually with the VF SIM in is because (as it was explained to me) the VF SIM disables this menu - the menu still _appears_ it just doesn't do anything when you choose a carrier.