Welcome to the community.
Sorry to hear about the issues you're having with the Voicemail.
Have you called 222 to activate the service?
If you're still having issues, just head to your online account here and once logged in, click 'Plan and Device' and then 'Manage services' and just ensure Voicemail is set to 'On'.
Let me know how you get on.
Mark YForum Team
Need help changing SIM card? Find out more about activating your 4G ready SIM. 4G SIM Swap Help