I've recently come across to Optus from Kogan, and there's no visual voicemail on Optus.
But I am still getting the visual voicemail screen when I go to voicemail on my iPhone 6s.
In the past I have seen this screen disappear and be replaced with a call to the voicemail service.
Anything I need to do?
It's not clear from your post if you have activated normal Voicemail? Is that a feature you want? I'm not sure whether activating it resolves the deactivation of visual voicemail on your iPhone.
More info on voicemail can be found on our help and support page here: Set Up & Voicemail Options - Mobile
I've tried activating voicemail, deactivating it on the online portal, restarting the phone and reactivating it.
Nothing changes. The voicemail button on the phone doesn't do anything, I need to dial 321 manually.
Hmm, as you mentioned Optus doesn't support Visual Voicemail so it's hard to try to assist with this one.
Perhaps Apple chat support can assist further.
Let us know how you go.