He is correct, you will have to add a softphone, and that works well. I set my number that is assigned to my ATA for simi-ring to my softphone. I find it better to keep the ATA connected, as the Vonage network outage forward nubmer is not very reliable in my experience.
Also if you have a 800 number or other numbers with Vonage you can use the DID feature of 3CX to decide where to route those calls.