I have been surprised to see how many folks are having trouble using their Vonage service with Express Talk.
Please keep in mind that, to the best of my knowledge, the only Vonage product that can be used with Express Talk is their "Softphone" line. If you have Vonage's standard VoIP service, your phone line is associated with the networking hardware they provided at the time of purchase. Vonage will honor requests to move such a line from one device to another, but does not honor requests to convert "standard" VoIP lines for use with a Softphone installation.
If you specifically purchase a Softphone line from Vonage and wish to use it with Express Talk rather than the bear-bones software Vonage provides, it shouldn't be a problem at all. I have been successfully using a Vonage softphone line with the current release of Express Talk for several weeks -- for whatever reason, I haven't experienced any of the trouble discussed in this thread.
Aside from the phone number (SIP username) and password information available in the "Manage Your Softphones" area of the Vonage website, there is one extra piece of information that Vonage's softphone already knows but which Express Talk will need you to manually enter. When configuring the SIP proxy server in Express Talk (or Axon -- with which it also works fine for me), you must enter the following:
This isn't a particularly hard-to-find piece of information, however, and I'd be shocked if the trouble others are having were due to using a different value for this setting.
It's also worth mentioning that, by default, Express Talk starts using ports for RTP at port 8000. All current Vonage devices seem to start with port 10000 instead, so I manually made this adjustment in my Express Talk settings when going through the initial configuration. In my case I'm using a static IP address and a router on which these ports had been manually configured, so I had no doubt as to which ports were actually being used.
In any case, I hope some of this information was helpful to someone!