I too have been getting calls at a work phone. On the phone display it says "VONAGE", so I goggled the number and found all kinds of complaints about people getting called.
So today, I called the "real" Vonage 800 number and told them what's been happening. They searched their customer database, and they said they don't own that number, neither for the company or for their customers. So my conclusion is: it's a SCAM! And Vonage is taking the blame for it.
Vonage suggested I call my phone provider and have them block that number.
My mom's caller ID says it was Vonage and this man on there was trying to get my mom to sign up with Vonage and she kept telling him she couldn't because she wasn't fiancially stable and she was on Hospice Care and she couldn't afford it right now and he just kept on asking so I got on the phone and told him she wasn't interested and thankyou for calling and then I hung up the phone while he was still talking. I am really getting tired of these people calling from different places wanting my parents to sign up for something and my parents tell them no so many times and they just keep calling.