Much better & simpler method: simply phone your own voicemail service. Then one of the options will be to ‘record a voicemail’; after which you can enter the phone number, record & send voicemail.
3. Call your voicemail system. Call your voicemail account by typing in its phone number with the keys on the dial pad, and then pressing the green Call button.

Some carriers require you to reset the voicemail password as a security precaution when you get a new iPhone. Sometimes, updating it manually online or by calling customer support can reset your iPhone’s connection to the voicemail server and fix the problem.
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.