Most businesses will automatically play a recording if you try to call their office when they are closed. An after hours announcement is a professional way to notify callers of important details, rather than making them wait for the call to failover to a voicemail box or other destination.
Besides, to update Carrier settings, go to Settings > General > About. If any carrier update is available, the pop-up will ask you to update it.

How do I change my voicemail greeting on Android? 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. Access your main voicemail menu. Listen for instructions to enter your password. From the main menu press 3 for Personal Options. Press 2 for Greetings. Press 1 to Change Greeting. Press 1 to Record personal greeting or press 2 to select the standard greeting. Tap on Call settings and select Voicemail. Tap on Voicemail service and select My carrier or My operator. Tap on Setup, select Voicemail number and type in your voicemail number. Tap on OK in the Voicemail number changed popup.
The first thing your callers should hear is the name of your business or organization. This assures them that they have dialed the right phone number and keeps them on the line. You may also want to include your address if there are multiple companies with the same name.