I used this web-based calling service for a few years while overseas. It is very convenient in that you can use your computer to make a call to two separate parties, as well as using a special toll free number when abroad, in order to request your calls. This is very helpful if you are not able for whatever reason to get online. The only difficulty is that the website was sometimes difficult to navigate and your user name can never be changed: minor problems for a decent and helpful service.
I was an off and on user of Call2 for 10 years. The service sometimes works well. At other times, it fails completely. Recently, they closed my account, because I hadn't used it for 6 months. Customer service was a disaster when I tried to re-instate my account with a "we don't care we don't have to" attitude. I'll not be using them any longer.