Unwanted call forwarding

I’ll try. As you have been told, call forwarding is a carrier service. The setting on the phone just sends a message to your carrier, and the carrier’s network gets the message and implements it. You can also enable call forwarding by calling your carrier and requesting it, or log into your carrier’s website and turning it on there. So it’s possible that someone called your carrier, impersonated you and asked for call forwarding. Or they hacked your AT&T user id and password and turned on the feature. What you need to do is change your AT&T password that you use on their website, and add their 2 factor authentication feature. This will also protect you from someone calling AT&T because the added authentication also works for phone contacts.

Just because call forwarding shows on the phone doesn't mean it was set by the phone. When you go to:

Settings > Phone > Call Forwarding

you will see a spinning gear on the right side of the screen for a few seconds before the Call Forwarding switch or number shows. This is the phone communicating with the carrier to get the Call Forwarding info. The same is true for the Call waiting and Show My Caller ID settings. The info is at the carrier, not on the phone. It can be hacked with no involvement of you phone. Do a Google search for "SS7 Hacking". SS& is short for:

Signalling System No. 7

https://en.wikipedia.org/wiki/Signalling_System_No._7

Your Call Forwarding may have been set by mistake at the carrier but more likely done on purpose to generate revenue at Saint Thomas. When a foreign country receives calls the originating carrier pays them a fee to complete the call. Some small countries have corrupt phone companies that trick people into calling their country so they can collect these fees. One trick is to leave a message with a number to call. When the number is called, the person who answers will try to keep the caller on the line as long as possible, so the charges will be high. Saint Thomas is ideal for this because it uses a US area code (340) so people don't realize it's an international call. In your case, they hacked your carrier's Call Forwarding to generate the calls.

The main thing people have been trying to tell you is that it shouldn't be possible, and the carrier is the one that controls call forwarding. No one here could tell you how it happened if no one had your phone in their hands to make the manual change. Since the carrier controls call forwarding, unless you manually change it, I'm very surprised they couldn't answer your question.

One other thing, the telephone receiver icon with the arrow in it is what indicates that call forwarding is on, so if you see that again, you will know something happened. You need to turn off the switch for Call Forwarding. If you are truly concerned there is an issue with your iPhone, then restore the device to factory and set it up as new, not using any backup. That would remove anything nefarious that may be on your device.

You can only change iPhone's settings directly with iPhone in hand,

unless your iPhone is jailbroken (modified iOS).

As for call forwarding, I think you can request the carrier to make the changes.

FWIW, this is the first post about "auto" call forwarding since I join the forum in 2011.

I would say it's a glitch on the carrier side. Make sure the change is done and tested.

Call forwarding is part of your carrier service and they got no clue?

Certainly it was not hacked which is what you implied.

iPhone options cannot be changed remotely.

As said call forwarding is carrier feature, you can only do it on your phone when they allow it.

For information about conditional call forwarding (if available from your carrier) when the line is busy or not in service, contact your carrier for setup information.

Read here: Manage incoming calls on iPhone - Apple Support