This will forward all inbound calls to the number you specify, it lasts until the feature has been turned off.
As soon as a number is entered, the calls will start forwarding. The example shown is forwarding calls to a number
that doesn’t exist:
You will always know the iPhone is forwarding calls because of the small phone icon in the title bar with an arrow
pointing out of it:
Switching off forwarding is even easier than turning it on:
The phone icon should disappear from the title bar, and the iPhone will accept inbound phone calls and ring again.