iPhone ringer volume lowers itself to zero despite settings

Mine started doing this as well but I have not looked into it (I will procrastinate tomorrow)but the ringer volume does not sound.

Where I would check is:

Is your phone's iOS up-to-date?

Settings>General>Software Update

Are you connected to a bluetooth speaker? If so remove the device, or shut of bluetooth if you're not using it

Settings>Bluetooth

Do you have Do Not Disturb or sleep settings on?

Settings>Focus

What is your haptics for ringtones and alerts set to?

Settings>Sound & Haptics>Haptics

At the moment I'm using the option for LED flash alerts, and there are several other options as well if you can't get the phone to work. I'd also try resetting the device to default but backup first before you try it

Accessibility features for hearing on iPhone - Apple Support

another article was posted on this forum in 2023, it may help

iPhone calls going straight to voicemail - Apple Community

Jakab23 wrote:

I’ve tried multiple iPhone models and iOS versions over the last 2–3 years, and it happens consistently across devices.

The iPhone ringer volume automatically lowering itself to zero is not something I’ve ever encountered in all my years using iPhones — starting way back with the iPhone 3GS. Through numerous iOS versions and hardware upgrades, that particular issue has never cropped up. My usage patterns have remained fairly consistent, and I’ve never had to worry about missing calls due to the ringer mysteriously silencing itself. So if it’s happening with your devices, it’s either a setting or automation that you may have, or a hardware quirk specific to the devices owned by you.

My phone does the same and calls go straight to VM