Hmmm. I normally do it straight from my Gmail page via the phone icon above the chat area.
It makes the the same graphic you've shown pop up, with the keypad and everything (though mine has a dollar sign rather than a £ sign).
When I try to make a call, I get this:
Which is what I normally get when I don't use an IP proxy.
You wouldn't think it would make any difference if you do it through Gmail or Google Hangouts as it all seems to be the same software, but maybe I'll try it through Google Hangouts, too. I've also got my language setting on English (US), which apparently back in the day was the only way they used to tell if you were in the States or not, so all you had to do was set that on English (US) and the calls went through....but then they started doing IP tracking and that didn't work anymore.
Would be great not to have to use HotSpot Shield - it's a real pain with all the ads, and a memory hog to boot!