Send your incoming calls to another number when you can't answer your iPhone.
On older iOS versions, "Phone" sits directly in the main Settings list. Call forwarding is a carrier feature, so it only appears for GSM carriers like AT&T and T-Mobile — Verizon users dial a code instead (see the last step).
If you don't see this option, your carrier handles forwarding with star codes instead of a toggle — skip to the last step.
Toggle Call Forwarding on, tap "Forward To," and type the full number (with area code) where calls should go. A forwarding icon appears in your status bar while it's active.
Open the Phone keypad and dial *72 followed by the 10-digit number, then press call, to turn forwarding on. Dial *73 and press call to turn it off. Check your carrier's site if those codes don't work.