Once you have your new iPhone in hand and you are learning how to use it, one of the first things you may want to do is record a voicemail greeting. You have the option of using a default one that tells the person what number they have called, but you may elect for a more personal option.
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?

Twigby sells the phone, and the user manual may be found at twigby.net/manuals if you purchased it from them.
All sorts of funny issues can occur when your phone hasn’t had a reboot in a while. Rebooting allows your phone to clear out and fix any issues. So try switching it off for at least five minutes, then see if the problem is resolved.