"this # does not accept unknown callers"
That line indicates that the person you're calling does not accept calls from anonymous numbers, which means you called them by dialing *67-xxx-xxxx when you dialled the number. I'm sure you knew that much though.
I had the same issue calling my sister, who is on Comcast, after she changed her number, for a couple days, and then suddenly, it just started working correctly.
She has had that issue when calling me from her cell phone, and has ever since I got Vonage, at the beginning of this year. She has one of those freebie 200-250 minute phones. I do have my Vonage adapter set to block anonymous calls, but she still can't call, and get's that message, even with it unchecked.--
The Firefox alternative.