To set up your voicemail, press *98 from your home phone or call the retrieval number you received with your welcome letter. Voice prompts will guide you through the rest of the steps.
The cause may be down to the phone or the voicemail application. To clear the phone’s app data:

As with most technology and applications, experiencing glitches at times is normal. We may not be able to pinpoint the cause of why you are not able to delete your voicemail messages, but here are three things you could try via your Android phone to solve 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.