VoIP providers can set caller ID up to work one of two ways.
1) The provider sets the Caller ID information when sending the call to the receiving party
2) The provider obtains the Caller ID information from the client software and passes it on
Preferably you want 1, thus no matter what happens on your end with Caller ID, when you call someone it will always appear as the Caller ID set with the provider
Theoretically if you change the display name in Express Talk's configuration, it may send that on to the provider if they use the #2 method, however I'm not sure about that. If you change the display name and nothing happens, I believe to my knowledge that you are out of luck, and unless the provider can set it up so it sends the Caller ID information instead of trying to retreive it from Express Talk than I don't think you'll be able to solve the problem. Most likely the client they provide is set to send the appropritate caller ID linked to your account automatically, where as third party software is not.