I have changed from orange pay as you go to giff gaff I have transferred my old number successfully but when I put in my new sim card the message is SIM card not valid I cannot use the phone at the moment I have bought a £10 goody bag
You phone is probably lock to Orange, you need to get it unlocked to use giffgaff.
Most networks lock ALL pay and go phones to prevent you moving, causing them a loss of money where the phone is subsidised
make sure your mobile phone is unlocked, or your sim could have a fault with it
the phone is not unlocked, try it in another phone