My visual voicemail isn't working
First, make sure your APN settings are updated to wap.vodafone.co.uk - this is the most common cause of issues with visual voicemail
To update your APN settings:
Go to settings
Select mobile data
Select Mobile data network (or settings)
Select Cellular
Select Cellular data network
Alternatively, you can tap on reset settings at the bottom of the APN page, which will revert all the APN fields to default
Other steps to try
Turn airplane mode on, wait a moment, then turn it back on again
Restart the device
Removing Visual Voicemail, then add it back to your account. Call 12152 to remove it. Finish the call, then call 12151
Make sure you have the correct number in the My Number section in Settings under Phone
Text STATE to 100121