So after reading about iOS10s voicemail transcription service, it becomes apparent that the feature only works on networks that support visual voicemail (Initially I'd thought that Siri would act as your voice mailbox and answer calls for you). 

Doing a bit more Googling I found this article.

 Vodafone spokeswoman Emma Carter said it would launch visual voicemail on iPhones early next year and would have an app available for Android smartphones "a little later". However, text transcripts were not a feature it was looking to provide, she said.

 But 2degrees appears as though it may be first across the finishing line. Spokeswoman Charlene White said it had been working on visual voicemail for a while and expected to offer it "within the next couple of months".

The article is dated 07/09/2014, coming up close to two years ago. Has any update been made on the above at all? It seems to have been largely forgotten about. The closest I can recall is the thing Spark does where they send a text with a number you call that takes you direct to that voicemail, but this is hardly visual voicemail tbh.