Unless you have some particularly wierd telephone system, you should be able to have your number transferred. I believe it was FCC regulation years ago that mandated that.
From the Vonage FAQ:
Yes. In most cases you can keep your current phone number. You’ll be able to check if your number can be transferred using our Port Number tool found in the tools module near the bottom of every plan page. If your number is transferable, we’ll help you bring it over to Vonage. And because transfers usually take about 7-10 business days, we’ll give you a temporary phone number so you can start using your Vonage service right away.