My former company tried to force me to use Vonage, I said NO WAY. I'm in sales and their quality was shitty to say the least. I recommend and use Broadvoice.com for one of my lines, as it has great flat rate plans. Also, I am NOT an affiliate or aything of them, just been using them over 5 years without an issue.
I use BroadVoice also and have been happy with the service. I have 2 lines with them.