I called into my voicemail, and tried to change the greeting to no avail. After a few attempts, I realized that I had had an "extended absence" greeting in effect. Once I switched to a "custom" greeting, I was able to record and activate a custom greeting.
Website: https://ipofficekb.avaya.com/user/ipoffice/mergedProjects/1408/gettingyourmessages.htm

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.
By using the reset network settings, an effectiveoption to solve network related issues, you can fix allthese problems by simply resetting the networksettings of your iPhone as it will clear all the networksettings, current cellular network settings, saved Wi-Finetwork settings, Wi-Fi passwords, and VPNsettings How do you reset your voicemail greeting? From the Home screen, tap the Phone icon . Tap Voicemail then tap Greeting (upper-left). Greeting islocated in the top-left corner of screen. Tap Custom to record a greeting. Enabled when a checkmark ispresent. Tap Record to begin recording the custom greeting message. Tap Stop to end recording then tap Save. How do you change voicemail settings on iPhone? Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can recorda new greeting. Tap Done to save your greeting.