Red dot on voicemail icon won't go

I have an iPhone 5 and I recently changed Network provider to BT from EE to SIM only contract, however ever since it went live the red dot on voicemail icon will not go off even though I have no voicemails.

I have tried resetting Network settings, adding my own voicemails and deleting them, changing my voicemail PIN but nothing works.

There is plenty of storage left on my phone and I am up to date with iOS 10.2.1.

Can anyone help?