Can you change over numbers on an IPhone manually?

Many people that have an iphone will know that if you go to

settings>phone>it says number

can you swap over the number from one phone to another

for example

my mum has an iphone 3gs and just brought an iphone 4 can she use the settings method above to switch the numbers over without anything going wrong?

If the phone has a sim card, you swap those to change the number. In the US, AT&T and T-Mobile use sim cards. Outside the US, most carriers do. Iphones use mini or micro sim cards, while other phones may use full size cards.