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.
Android phones can access voicemail by holding down the 1 key on the dial pad until the phone dials your 10-digit phone number. You will automatically connect to your mailbox and be prompted to enter your temporary password; this password is the last 4 digits of your phone number followed by the # key. Start the Google Voice app. Tap the three horizontal lines at the top left of the screen (sometimes called a hamburger menu), and then tap “Settings.” In the Voicemail section, tap “Voicemail greeting.” Tap “Record a greeting.” How do I access my voicemail? To call your voicemail on an Android phone, simply open your phone’s dial pad and hold your finger down on the “1” key. You can also call your voicemail from a different phone by calling your own number and tapping the pound key. How do you check your voicemail?

To change your voicemail message on iPhone: 1. On your home screen, tap Phone. 2. At the bottom, tap Voicemail on the menu bar. 3. Tap Greeting in the top left corner of your screen. 4. Tap Custom if you want to record a custom greeting. 5. To record a new greeting, tap Record. Tap Play to listen to it. 6. Lastly, tap Save in the top right corner.
4. Toggle Cellular Service on and off. You can turn off and then turn on Cellular Service from Settings or Control Center. Launch Settings → Cellular → Toggle off Cellular Data → After 30 seconds, toggle it back on.