We managed to discover some of the most common causes that will trigger this particular issue. We have done this by looking at various user reports and the repair strategies that they used to get the issue resolved. Here is a shortlist with common scenarios that have the chance of triggering this particular error message:
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 ...

For those who are satisfied with the Default greeting for their iPhone voicemail setup, you can skip this step by clicking ‘Done’.
To improve a a business voicemail greeting, keep these eight rules at the forefront of the creative process: Avoid turning customers off with overused and impersonal phrases like “your call is very important to us..." Avoid leaving customers unsure by not immediately telling them the business, department, and/or person they’ve reached. Avoid leaving customers confused with too many details and complications; just keep it simple. Avoid messages longer than 25 seconds. Do apologize for being unavailable at the moment. Do invite the caller to leave a message. Do tell the caller when they can expect a return call and actually follow through within that timeframe. Do tell the caller about any applicable alternative options of contact and information- website, live chat, email, social media, or emergency numbers. Voicemail Greetings 101