Welcome to the Community Forum and thanks for your post. I'm sorry to hear that you are experiencing a problem with voicemail service since upgrading to 4G. Has this been resolved for you since your post?
If you are still having a problem with this, please see our online Help & Support page on this: Virgin Mobile Voicemail.
If you continue to experience an issue with this, please let me know and I'll be happy to look into this for you.