Call Forwarding is a carrier-specific feature (like Personal Hotspot). If you've got an iPhone on a network which supports call forwarding, you can set it up very easily and forward all your incoming calls to a particular number.
Not a lot of people actively use call forwarding on their iPhones but it turns out into a very useful feature if you are busy and can't take calls and would instead divert them to another number.
If you have ever wondered how to forward calls from iPhone to another phone, it is pretty simple and in fact, just too easy.
However, before you get started, you'll have to find out from your carrier if they let you in on that feature. Some carriers have charges for call forwarding while some don't. It's country specific too. So before you get started, check that info out with your carrier. Otherwise, it just doesn't work.
How to setup call forwarding on iPhone:
Step 1: Go to Settings
Step 2: Scroll down and tap on ‘Phone‘
Step 3: Tap on ‘Call Forwarding‘ and toggle the switch to ON
Step 4: The moment you switch it on, ‘Forward to‘ will appear. Tap on it and enter the number to forward the calls to
Step 5: Once the number is added, your call forwarding will be activated
There aren't other options to select or tweak here. It's just plain but it works.
You should see a sign (like it's shown in above screenshot) on the status bar. This is a confirmation that your calls are being forwarded to a specified number.
To a caller, there's a seamless shift so no one really knows where the call is actually going. For instance, if you happen to forward it to your home number, callers wouldn't know till someone picks the phone back at your home and tells them that you've forwarded the call. Also, if you forward the call to an invalid number or a number that is no longer working, callers would only assume that your number is not working.
So there you have it. It's pretty easy to forward calls from the iPhone.