Click "Cellular" and choose "Add Cellular Plan.". From there, you have the option of manually entering the information or scanning a QR code that your carrier has provided. You caslots comn select the number you wish to use for data, calls, and messages after entering the details for your second SIM card. In the Settings app, tap the "Cellular" option to quickly switch between the two numbers after configuring your iPhone 15's dual SIM. From there, you can choose the number you wish to use for data usage, message sending, and phone calls.