The *78 feature should override this permanent diversion, and an incoming call to the landline phone should be immediately bypassed without ringing (or perhaps ring once, like it used to do before NBN) and diverted to the mobile and ring long enough (more than the default 6-8 secs) to be picked up by the recipient or picked up by the mobile's ...
**Important: Please read entire article** It can be frustrating to experience delays with your voicemail. That delay may occur for a variety of reasons – most likely due to issues outside of the YouMail app. Data connection, WiFi connection, third party applications or device OS can all conflict with the retrieval of your messages if configured incorrectly.

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Open the Phone app. Tap the Voicemail tab in the lower right-hand corner of the screen. If Visual Voicemail is not set up on your iPhone, you'll see a button that says, Set Up Now. Tap that button. Enter, then confirm a custom voicemail password. Tap Custom if you want to create your own custom greeting when someone reaches your voicemail box.