Which display card should I buy?

With this type of lock it's often possible to buy a cheap pay-as-you-go handset, and replace the SIM card with your current subscription SIM card from the same service provider. In this way you can buy a cheap subsidized handset, while retaining your existing subscription. However, some service providers use a different service provider code for their pay-as-you-go handsets, so it won't recognize a subscription SIM card as being from the same service provider.