Lili, have you tried checking the voicemail settings? Call your voicemail and go to settings. Check if there's an option to adjust number of rings/time before voicemail kicks in. If there's none, I think the only option you have left is to call their customer service.

Carl, it's a UMX, via Lifeline. It's their latest version I think. Absolutely nothing in the users manual about it. 

Hi Lili. Which carrier are you with? I think that may depend on which carrier you are with. You may also try to contact your carrier to know if you can make adjustments.