It comes with a SIM card. Yes Iphone 12 comes with the Sim Card included you don’t have to buy anything my phone came with the Sim Card and at best buy they helped me install it they set it up and transferred everything from my old phone to the new phone when I left the store phone was ready to use.
Open the Phone app . Press and hold “1” to call your voicemail. Enter your PIN and press “#”. Press “*” for the menu. Press “4” to change settings. Press “1” to change your greeting. Follow the recorded instructions. How do I change my voicemail message? From the Home screen, tap the Phone icon . Tap Voicemail then tap Greeting (upper-left). Greeting is located in the top-left corner of screen. Tap Custom to record a greeting. Enabled when a checkmark is present. Tap Record to begin recording the custom greeting message. Tap Stop to end recording then tap Save. Call *86 (*VM) from the handset or the 10-digit mobile number from another phone. Follow the instructions for choosing the language options then press the # key to confirm the choice. When prompted, enter the 4-7digit password then press the # key. When prompted, say the signature (or name) then press the # key. Open your Android’s Phone app. It usually looks like a phone receiver at the bottom of the home screen. Press and hold the 1 key on the keypad. Tap Add number. Tap Service. Tap My carrier. Tap Setup. Tap Voicemail number. Type your mobile phone number and tap OK.

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In your Phone app, navigate to the number pad. Pressing and holding the 1 key will dial you into your Voicemail's main menu instantly. You can also call your own number or select Voicemail from the list of options under the three-dot menu in the app. If this is your first foray into the matter, the friendly voice on the other end will guide you