Follow the instructions and press that number. You will be asked to delete your current Voicemail greeting. Click on the ‘Yes’ button that appears on the screen if you want to remove that. You can save your new recorded Voicemail greeting or can use the default recorded option. From the home screen, select the Phone app. Note: Alternately, you can set up voicemail by selecting the Phone app then selecting and holding the 1 key. Select Keypad, then select the Visual Voicemail icon. Select Start. Select Ok. From any Home screen, tap the Menu key. Tap Settings. Tap Call settings. Tap Voicemail. Tap Voicemail again. View the voicemail number. Press and hold the 1 on your device’s dial pad to call and connect to your voicemail. Press the star * key to access the main menu. Press 4 to access the mailbox options. Press 7 to change the language of the voicemail prompts to either Spanish or English.
One of the best and quick fix to this issue is to turn off Wi-Fi for once and switch to Cellular data connection and then try to change the default greeting to custom on iPhone.

We provide free recorded voicemail greetings: The best way to greet your callers—and create a stunning impression—is with a professional, pre-recorded voice message. AccessDirect’s professional, pre-recorded voicemail greeting tells customers that you are a serious business—every time they call.
Generally, these greetings should be brief for an individual. State your name and a query for the caller's information. You can also include a backup number for people to reach you if you like. For an organization, it's a good idea to announce your standard office hours. Dial *98 on your OnSIP-registered phone. Enter your voicemail box number. Enter the PIN/password for your voicemail box. Press 0 for voicemail box options. Press 1 for unavailable greeting.