I do not use Vonage or UC-One, but I would try these ones:
"Dial phone" script step (Windows only)
"Open URL" with "tel:<yourphone>"
Send Event with send phone to UC-One
Which operating system is in use?
I can only really talk about a project we did for a windows environment using Splicecom voip telephones with maximiser/Navigate software. We had a great deal of problems getting a button to work correctly to dial a contact's phone number using the 'DialPhone' script step.
In the end, it was a realatively minor issue in that we needed to download and istall TAPI drivers for each pc and set up each pc's 'Phone and Modem' settings in Control Panel to use the TAPI drivers..
First port of call, if you are indeed using Windows, is to contact Vonage and UC for their TAPI drivers.