How can I figure out why calls to one iPhone ring on another?

Start by checking whether both phones are signed in to the same Apple Account. On each device, look in Settings at the account name at the top, then check the phone number and email addresses listed for Messages and FaceTime. If one person's account is signed in on the other phone, that could make calls or messages appear on both.

Also check Settings, Apps, Phone, Calls on Other Devices on the phone receiving the extra calls. Turn off access there if it is enabled and not wanted. Avoid signing out or removing accounts until you know which account holds the contacts, photos, and other synced data.

First pin down what kind of call it was. A regular cellular call, a FaceTime audio call, and a call forwarded by the carrier can behave differently. Try a test call to the number and note which device rings, then check call forwarding in the phone settings and with the carrier if needed.

I would not assume the number has been moved or ported just from one odd call. Check the simple account and call settings first, and make one change at a time so you can tell what fixed it.

For location sharing, use the built-in Find My app and share the location with the other person's contact. Sharing a location does not require signing one person's Apple Account into the other person's phone, and it should not require changing where calls go.

Before trying that, confirm each phone is using its owner's account. If the account settings look confusing, pause and get help from Apple Support rather than deleting anything. A careless sign-out can affect synced data.

Keep the fix small: check the account at the top of Settings on both phones, then check Calls on Other Devices and call forwarding. Take screenshots of the current settings before changing them. If the phones share an account, separate them carefully and verify contacts and photos are backed up first.