Welcome to the Community
Can you let me know at which step of the change you got this error message? Was it before or after you selected a phone number in your area?
Also, would it be possible to let me know what exactly the error message said?
When changing your phone number, all messages, greeting message and voice signature are lost. Is it possible that the message was prompting you to check your voice messages before completing the change?
Let me know and we'll take it from there