I am assuming you want to increase the time the phone rings before it's sent to voicemail.
That is a carrier setting, not a phone setting, but you can usually change it for most GSM carriers.
Open your phone's dialer and type *#61# and then press send (or call, or whatever your button says when you are trying to call someone). You should see the number of your carrier's voicemail service like +15551234567 plus a number like 15 or 20. Write that number down.
Then go back to your dialer and type **61*+15551234567*11*30# and press dial or send. Make sure the green number is the one you wrote down and not the fake one I typed here.
That will increase the time the phone rings to 30 seconds, which is usually the maximum. You can set it to anything in increments of 5 seconds.