There are 3 different ways to decline an incoming call on an iPhone. Declining a call will send it directly to voice mail. We’ll show you how it’s done with this detailed tutorial.
Use Sleep/Wake/Power Button
If your screen is locked when receiving a call, the screen only provides the option to answer the call. If you want to decline the call from the Lock screen, simply press the Sleep/Wake/Power button on the top of the phone 2 times in rapid succession. If performed correctly, the screen should fade off.
If the phone is unlocked, you will have an option to decline the call on the left and an option to accept the call on the right. Simply tap the red “Decline” option.
If you’re wearing your iPhone headset or any other supported headphones, you can simply press and hold the Microphone button for about 2 seconds. You should hear two beeps to let you know you have successfully declined the call.