Voicemail

I’ve upgraded to the iPhone XR in August. Backed up to iCloud before the transfer. I’m able to call into voicemail and that works fine. When people call me, they cannot leave a message as the prompts says “ sorry that mailbox is invalid”. I’ve reset network settings several times, taken it off WiFi, did a compete reset, and still doesn’t work. My carrier states it may be an apple app issue as they cannot resolve. Please advise.

Thank you!