Taken from the Vonage Terms of Service document (available from their website):
11.2 No Warranties on Service.
We make no warranties, express or implied, including but not limited to, any implied warranties of merchantability, fitness of the service or device for a particular purpose, title or non-infringement or any warranty arising by usage of trade, course of dealing or course of performance. In addition, we do not warrant that the service or device will be without failure, delay, interruption, error, omission, degradation of voice quality, or loss of content, data, or information. Neither Vonage nor its officers, directors, employees, affiliates or agents, or any other service provider or vendor who furnishes services, devices, or products to the customer for our service will be liable for unauthorized access to our or your transmission facilities or premises or equipment or for unauthorized access to, or alteration, theft, or destruction of, customer's data files, programs, procedures, or information through accident, fraudulent means, devices, or any other method, regardless of whether such damage occurs as a result of Vonage's or its service provider's or vendors' negligence. Statements and descriptions concerning our service or device, if any, by Vonage or Vonage's agents or installers are informational and are not given as a warranty of any kind.
And also this:
6.5 Fees upon disconnection.
(a) Disconnection by you. If you attempt to disconnect service for one or more of your lines before the end of your current service term, you will be responsible for all the charges for your current term that are applicable to the disconnected lines(s), including unbilled charges, plus a $39.99 disconnection fee per line, if applicable, as provided in Section 8.7 below. Cancellation is effective on the date you cancel and paid charges are not subject to refund. If you cancel your service before the end of your billing cycle, there are no pro-rated refunds. [...]
You have nobody to blame but yourself. Read what you accept.