Here is my experience with Vonage:
Vonage will not be the cheapest service out there but it works pretty well and it's cheap enough for my needs.
I got Vonage because I was moving out-of-state and wanted to maintain my landline number. I plan to keep this landline # for the rest of my life or until I decide that I no longer need / want a landline. (Sometimes you need to give people / companies / organizations your phone number but you don't want to give them your cell number, hence a landline is handy for that.)
I have the Vonage World Plan. It's listed as $25/month which means that it's actually $36/month after taxes, fees, etc.
That includes all the standard stuff (voicemail, caller ID, blah blah blah) with unlimited calling in the US (to landlines and mobiles) and unlimited calling to landlines in the UK and most (if not all) of Western Europe and a whole slew of other countries (but not all countries). These details and the rates for calling cell phones in each country are clearly identified on their website.
Also, with the Vonage Extensions, you can setup one number (ie your cell phone) to "freely" leverage your VOIP landline's international calling plan. What that means is that you can use your cell phone to make international calls to landlines in Europe for "free" or to Euro cell phones at Vonage's reasonable rates. "Free" is in quotes because you are actually consuming minutes from your cell phone's monthly plan as if you were calling a US land line. (At least this is how it was a few months ago. Even though they have iPhone and Android apps for this capability, the app was still dialing a US access number behind the scenes as opposed to using the cellular data link or, better yet, a wifi connection. Braindead, but true. I haven't checked recently if they have improved upon this. However, what this does / did mean is that you cannot use your Vonage Extensions iPhone app overseas with wifi to freely call the US or European landlines
Another nice feature is that when someone leaves you a voicemail on your VOIP line, you can receive an email that includes an automated transcription of the message (works pretty well if people speak good English; you can get some pretty funny mis-translations sometimes; for many foreigners, the service simply doesn't understand their English) along with a .WAV file so that you can actually listen to that message. Hence, if someone leaves a msg on my VOIP line when I'm out and about, my iPhone receives that email and I am in the loop. (Yeah, you can play around with all that forwarding shenanigans but that it not how I desire to use this VOIP number.)
I am a happy enough Vonage customer. As are various friends.
The call quality is usually solid but every now and then a call degrades and I simply hang up and call back.
That said, I have this European NYU professor friend who is really into his technology and finding the latest, greatest, cheapest solutions for various stuff. A few years ago he was on a mission from God to optimize his phone budget. Because I would tolerate him, he used to call me way too frequently and, upon answering the phone, I'd hear "How do I sound ? Can you hear my clearly ? Any static ?" He would be trying out some new VOIP provider. He went through a dozen of them. He would tell me about these insane rates ($19.99 or $12.99 per year for unlimited international blah blah blah). They all sounded oh-so-intriguing but he never seemed to stick with one. He was on to the next. Why ? Well, of course, he'd stick with one if they met his needs on an on-going basis, so ...
It was simply never worth the hassle to me to try to further optimize these costs.
And you know Chambo ... he's an optimizer.