When going to Voicemail, all callers get User Busy indication or busy signal.

My problem has to do with not receiving any voicemails. I have no problems sending and receiving phone calls and text messages. The problem is that if I let the phone ring so it goes to voicemail, callers either get a "User Busy" indication or a busy signal. I have also tried to call my own voicemail from my iPhone by dialing my own number and also long pressing 1. Both times I got the "User Busy" indication and had no way to access my voicemail. This problem may have been going on for sometime because recently I have noticed that I have not received any voicemails from people calling while I was unavailable to answer. These include recent calls from doctors and dentists who have left voicemails in the past.

I was wondering if anyone else has had this problem and if any solution is available. I have already Googled many sites about this and none of them worked. I even reset my network settings, turned of all focus and do not disturb features, and even factory reset my iPhone and restored from backup. None of these worked so I am thinking this is a network issue with my account or something. If anyone can help, it would be greatly appreciated.

My phone is iPhone 13 with iOS 16.6.1