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Firstly you would need to deactivate HulloMail to stop HulloMail from handling your voicemail.
For iPhone, please dial the 'HulloMail Deactivate' contact from your iPhone contacts.
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.
For iPhone, please select; Menu -> About -> Service Termination -> Close Account
For Android, please select; Menu -> Settings -> About -> Service Termination -> Terminate Account
Once the service has been terminated, please remove the app from your handset.