If the you haven't changed your personal greeting, a default system greeting will be played for callers. For example, "Please leave a message for John Smith. After the tone, please record your message. When you finish recording hang-up or press the pound key for more options."
When you reset network settings, all of your iPhone’s Wi-Fi and cellular network settings will be erased. This includes your Virtual Private Network (VPN) settings, your Bluetooth devices, and your Wi-Fi networks — so make sure you write down your passwords first! We do this step because it can be very difficult to track down the exact source of a software problem, so we reset all network settings.

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?
T-Mobile: T-Mobile has a similar story to tell as Verizon. Instead of allowing you to set up a specific basic voicemail, it instead directs you to Apple to set up Visual Voicemail. I dont see anything that you have written when I try to set up voicemail on my new Iphone11. I tap on voicemail and the only option is 'Call voicemail' … then it doesn't accept my mobile number.