iPhone Visual Voicemail - can't be deactivated?

[rexone]

Ok, call me a Luddite but I do not like Visual Voicemail. It adds steps to the whole voicemail receiving & answering process & I find it is just one of those things that's a 'solution' to a problem that didn't exist.
Anyhews, putting that aside... when I bought my iPhone Xs a couple of years back VV was activated & I had to go through my carrier (Telstra here in Australia) to have it deactivated so I could have 'conventional' voicemail.
Two days ago VV has reactivated on my phone. I have not installed any iOS updates, it just switched itself on.
I have now spent hours trying to sort this out with my carrier. They insist they have done nothing and they now cannot deactivate it. Everytime they try they get an error on their side.
If I try to dial the number for 'normal' voicemail I receive a message saying I have no access.
Anyone have the slightest idea what is going on?
By-the-by I'm on iOS 13.6.1

 

[0128672]

It sounds as though you already understand that Visual Voicemail is activated with your phone service provider. For some carriers you can log into your online account and block that feature. I hope your carrier figures out how to help you with this.

 

[rexone]

It sounds as though you already understand that Visual Voicemail is activated with your phone service provider. For some carriers you can log into your online account and block that feature. I hope your carrier figures out how to help you with this.

It seems my carrier is as confused as I am about this.
They have escalated the issue as they cannot understand why they are unable to deactivate it from their side.
I'll post any info they provide but until then any more expereince or input is appreciated.