Verizon has a time limit, I think it's like 30 seconds, before voicemail kicks in. And there is no adjusting it. Some complain it's too short a time, and others feel it's too long (like yourself). There's no pleasing everyone.
Perhaps there's a third party service or an app that allows you to customize the time before it shifts to voicemail. I use You Mail which allows a lot of customization, but I've never looked into changing the time like that. I'll do that and report back.
** Edit with more info - You Mail works on a forwarding system - calls are forwarded and the time before it gets forwarded (or sent to voicemail) is set at the carrier level. So the short answer is no, you can't change the settings. 
Message was edited by: SuzyQ