#1 would be the service provided by your broadband provider, because the voice packets will recieve priority over the data packets while traveling on there network between your home, and the telephone network at there office...
#2 would be a company like vonage, a true VoIP provider, with an ATA adapter where you can use your standard phones, and not be required to leave your pc on, or be inconvienced by the limitations of the pc processing power while performing tasks and VoIP traffic at the same time..
#3 would be a soft phone provider like skype, cheap, decent quality, but most set ups require your pc to remain on to make or recieve calls, may have poor voice quality, depending on how much resources are being used in your computer by other application...
VoIP companies come and go, and some local phone providers offer VoIP but do not have a national presence... your best option not to get stuck like the people from sunrocket did, is to go with your cable modem providers VoIP, or verizon voicewing... you know that they will be in business for a while... otherwise keep with vonage, (although there have been some complaints on here lately... but then again people dont come on here to say, hey everythings working great...) if you dont mind a softphone, and want service really cheap, skype is the way to go... I think skype is more for a supplimental service then your only source of phone service..
i listed by quality.. if you want best for price just flip the list over and start from the bottom up..