Tap “Phone” on your iPhone’s “Home” screen and tap “Voicemail” to access your voice-mail settings. Tap “Greeting” and select “Default” if you wish to use the default greeting for your voice mails. Click “Custom” if you want to create a personalized greeting, and tap “Record” to say your greeting.
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 ...

We’re sorry to hear about this, if you can reach out to us on [email protected] with your details we can try to assist you further, however, we would need to confirm if you are able to manually access your voicemail through your phone. We can’t provide assurance or support for third party applications. However, if you encounter issues with manually accessing your voicemail through your phone we can assist with that 🙂
You might also consider giving them the option to skip your voicemail message completely by pressing another key (again, check with IT). You also want to make callers aware what you will return their call, instead of telling them that you will try to return their call--and let them know when you are likely to do it, if possible. If necessary, provide additional or emergency information in your voicemail greetings. My dentist, for example, lists his cell phone number for emergencies.