Firstly you would need to deactivate HulloMail to stop HulloMail from handling your voicemail.

iPhone

For iPhone, please dial the 'HulloMail Deactivate' contact from your iPhone contacts.

Android

For Android, please select from within the HulloMail app:

Menu -> Settings -> Voicemail Activation -> Deactivate Voicemail

Please bear in mind that the deactivation number is carrier specific.

Once deactivated you can then close you account.

iPhone

For iPhone, please select; Menu -> About -> Service Termination -> Close Account

Android

For Android, please select; Menu -> Settings -> About -> Service Termination -> Terminate Account

Once the service has been terminated, please remove the app from your handset.