Step 1) Go to your Phone app, and dial *#61# and hit Send

You’ll then see:

Step 2) Write down the phone number it tells you it forwards the Voicemails to (I blurred mine out) including the +1 before the area code prefix. For now, we will call this number “+1-ABC-DEF-GHIJ”

Step 3) Dial **61*+1-ABC-DEF-GHIJ*11*30# and hit Send.

(Hint: To get the “+” on the iPhone hold down the “0” button until it changes)

That last part “30″ in the number you dialed represents the number of seconds for the ring to go on for. You may only set this at a maximum of 30, and only in increments of 5 seconds. (ex: 20 seconds, 25 seconds, etc.) After hitting send, you should see this:

Dismiss it, and your all set. Now, to check it dial *#61# and hit Send again. Or you can always call from a different phone line. Hope this helps someone.