why unknown calls go direct to voicemail without ring

I am missing very important calls coming from undisclosed numbers which go direct to voicemail with no reason. Other calls come through without issue. This issue has been there for several months.

iPhone 13, ios 18.2

I went through guides on this topic, ensuring none of these was the case:

• no Do not disturb mode activated
• no airplane mode
• no headphones connected
• no network service provider issues
• no call forwarding
• no third party call blocking app

One other weird behaviour on that iPhone : every time I want to dial a new number which I never called before, I am prompted with an annoying message that the call can't go through because it's not in my contacts ! Then I have to go back, redial and then it goes through! Totally ridiculous, but perhaps related to the first issue.