Although the iPhone has many useful features, it won't do everything you want without a little customization. If you want to change the number of rings your phone emits before sending a call to voicemail, you can do so, but the ring length setting is not intuitive like other iPhone options. Using this hidden customization feature, you can set your iPhone to ring for a preferred number of seconds.
Power on your iPhone and tap the "Phone" icon on the main screen, located at the bottom left corner of the screen.
Press the "Keypad" icon at the bottom of the screen. Input "*#61#," then press "Call." A new screen is displayed. The fourth line reads: "Forwards to..." followed by a plus sign and an eleven-digit number. Write this number down.
Tap "Dismiss" to go back to the keypad screen. Type "*61*," then enter the Call Forwarding number that you wrote down. Press "*11."
Enter "*," then enter the number of seconds you want the iPhone to ring before sending the call to voicemail. Type "#" after the number of seconds. A"Setting Activation Succeeded" message appears. This indicates that your new settings are changed and saved.