Why do I lose some phone numbers when I switch my sim card from one phone to another?

Some are there and some aren't. Am I doing something wrong?Why do I lose some phone numbers when I switch my sim card from one phone to another?
it might be because some numbers are stored on the phone and others are stored on the sim card...thats what happend with my phoneWhy do I lose some phone numbers when I switch my sim card from one phone to another?
I bet that when you return the sim card to the older phone you will be able to see the contacts that were missing. Thats because you have saved some contacts in the SIM card (those are the ones that you can see on both phones) but you also saved some contacts on the actual phone, so they wont go to the other phone when you change the sim card.