I recommend Verbonden.nl (for quality and service).
I travel a lot and I work abroad often. I've been using the Dutch voip-provider Verbonden.nl for some time to keep in touch with local clients over there. I can honestly say it works very well for me.
The quality of the sound and connection is generally very good (provided that my internet connection is stable of course). Verbonden.nl is technically sound. I have no complaints there.
I love the fact that I can record and store phone conversations. Also, I like the fact that I can use it at the office on the desktop-computer and take it with me on my smartphone as an app.
Usually I take it with me. I don't use it often on the desktop. The app works just fine. The desktop use I will have to try more before I will review it. I plan to because it will allow me to make good use of the hardware that I have in the office (good quality speakers and microphone).
Only thing I would like to be better is switching between the two. It is a bit of a fumble in the online interface. Also I think the Verbonden.nl web interface could be a bit more intuitive. I hope they will work on that in the future.
But, having said that, once you have your stuff set up, you hardly ever need to go there. Things just work.
Service is also very good. Verbonden.nl has a service desk that is only one call (or email) away. You'll talk to someone who knows his business and who is happy to oblige. Good service. Hope they keep it up.