I have been playing around with this and called the vonage support 6 times and they don't have the answer for me.
I am using the vonage softphone to do the transfer, I get the ring on my cell phone, but once I pickup the call disconnects.
I tried different settings and it is not working for me.
I tried using the Vonage Softphone software to do the transfer and it works perfect.
So I know this is possible, please help!