iPhone 6 iOS 12.1
Visual voicemail no longer works possibly a result of upgrading phone firmware.
I have tried the following having researched online
Flight mode off/on
Resetting network settings
Tried voicemail at different locations
Best answer by Kevin
Had you been able to get visual voicemail working on the iPhone under iD prior to this recent update? See officially Three (nor ourselves) have not yet supported visual voicemail on iPhones. According to Apple, it appears to be exclusive to EE and O2 as of time of writing over here in the UK.