Why isn’t my phone ringing in on all calls

You might also be in Do Not Disturb mode, or in a Focus mode such as 'Driving', 'Work', 'Sleep', etc. which suppress all calls, other than from nominated contacts.

You may also have the setting to ignore unknown callers enabled. This prevents many spam and junk calls from ringing on your phone, but can also falsely block 'legitimate' calls from people who are not in your contacts list.

All these options are managed through Settings on your phone.

Check Settings -> Phone to toggle the 'Silence Unknown Callers' option.

Check Settings -> Focus to check your Focus settings and who can call during these times.

Check that your phone is in mute. Check silent Rocker arm above the volume up key.