when do i have to replace my sim card

Generally, you would never need to replace the SIM card. If you have an unlocked phone, you would need to replace it to use a different carrier. If you are having problems with calls, you may need a new SIM card because they can eventually fail. If you have a concern about your SIM card, contact your carrier for help.

Why do you think you would have to replace it?

When you change carriers or get a 5G device.