How do I change the number of times my cell phone rings until the voicemail message starts?

I have a Pantech Duo and you only get five rings until the voicemail picks it up. I want to extend it to five or six rings. How do I do that?How do I change the number of times my cell phone rings until the voicemail message starts?What service provider do you have?? I know with Verizon all you need to do is contact their customer service and they have the ability to change it in their computer system.