If calls are forwarded successfully when SimulRing is off or the cell phone is called directly, there are two approaches to solving this issue.
You can contact Vonage and tell them that when SimulRing occurs, the OCN (originally called number aka the dialed number) is not being forwarded to Youmail. We are more than happy to assist any Vonage engineers, so please submit a request if they need any logs or other information.
Alternatively, you can get your own Deposit Number. This deposit number will allow you to bypass the technical problems that cause your forwarding issue with SimulRing.