Welcome to the Community Forum, I'm sorry that you are experiencing a problem accessing your voicemail.
It sounds like there's a problem with your voicemail migration. Please try ringing the voicemail number +447953222222 on your phone and see if this allows you to access your voicemail. Please let me know.
Need help changing SIM card? Find out more about activating your 4G ready SIM. 4G SIM Swap Help