In the 3CXPhone API, the CallID is local to 3CXPhone, it's not the global CallID used by the 3CX server. Unfortunately there is no other property you can use as a replacement. But what you can do is using the 3CX Call Control API from the server to check the CallID for the call connected to a specific extension. This API can only be used from the server, so if you need to get this information from the 3CX client, you need to create a web service or something to communicate both machines.