Calls not going to voicemail

I have an iPhone 10X. When I am on the phone or i cannot answer my phone if busy, incoming calls do not go to voicemail. I used to hear a call coming through and it would go to voicemail. Callers now tell me it rings and rings and does not go to voicemail. Any idea why?

I have tried resetting Network settings, toggle airplane mode, restarting iPhone etc all to no avail

Weird thing is that if i call my own number from my phone it gives me the option to leave a voicemail as the caller is on another line.