Calls automatically go to voicemail

Some of my calls are automatically going to voicemail without ringing or any notification. I do not have Silence Unknown Callers on. I do not have Do Not Disturb on. The ringer volume is on. I do not have any apps on my phone that block calls. I just got a voicemail from a doctors office and I have the number saved in my contacts and added to my favorites yet it still didn’t ring. Is there any way to fix this? I’m using iOS 14.5.1