Highest quality from what I’ve heard is Vonage. There are less expensive options but you may sacrifice features. I had Comcast digital voice for ~30 a month, now I have Tmobile for ~10/mo. But now I have no web-voicemail and it can’t be used for faxing. There is a thing called magicjack which claims to give you home VOIP service for a year for ~$30 but there’s no human support, which is obvious given it’s low price. So, I think the leader in the industry is Vonage, but they’re not the cheapest.

U should try the ACN service. they are #1 direct Telecom provider in the world. not only there service is good…they also offer VIDEO PHONES! see the person ur talking to!

there are a lot of online companies, personally I think the best service is offered by RingCentral. You can check out more info and review of the best VoIP phone services here: http://www.voipphoneservices.net/

I use companies like CallCentric, Les.net, InPhonex, and Voip.ms.
These are all BYOD (Bring Your Own Device) voip services, which means you have to have enough tech savvy to provide and install and configure your own voip equipment. These companies are often used by business that install and maintain their own IP-PBX systems, but home users like myself can also use their services if you can install your own ATA (analog telephone adapter) and configure it yourself.
I configure my own Linksys SPA2102-NA adpater and have been using it for a couple years now with all of the services listed above, but have my DID phone numbers provided through CallCentric with very good results.
CallCentric also provides calling card gateways which allows you to make voip calls through their network by calling local gateway numbers located in major cities around the world. This way you get voip rates even when calling from a regular PSTN landline, or cell.