I sell VoIP service.
It will save most people a LOT of money. However, it is NOT for everyone.
I tell prospective customers that their fax machine will not work (sometimes it will, sometimes it won't).
I tell them it will NOT work with home burglar alarm systems (sometimes it will, sometimes it won't).
I tell them it will not work with their credit card terminal if they are using it as a dial-up machine (sometimes it will, sometimes it won't). Newer credit card terminals have an ethernet port if you don't mind the cost of the security screenings.
If you are pretty much alone in the home, then you can get away with a "1.5 Mb/s" ISP. If you have gamers or video watchers, then you are going to need at least "4.5 Mb/s" ISP. In your case you have cable, they are likely delivering 6 Mb/s or more. It's actually the upload speed that is the 'limit' to your happiness, but it seems no ISP ever advertises their upload speeds.
Oh.... if you use satellite internet.... VoIP will not work. (Baffles me because two-way video works just fantastic on a "1.5 Mb/s" satellite internet service, but I have never found a VoIP that works on 'consumer grade' satellite service. You need a C.I.R. service (committted information rate) which will be very expensive.)
I can't believe people are buying the magicjak idea. It costs here, in Florida, $21 a month to leave your computer turned on all the time... so any 'savings' by using their 'service' is eaten up by the cost of electricity. Plus if your computer goes down, well... no telephone !
I only support customers that buy a well known "ATA" (Analogue Telephone Adapter). It is basically a "sound card" and a 1 or 2 port (or more) router in a box plus the power supply to generate 20 Hz 90 volt ringing voltages and a bunch more functions. With a good ATA, you plug in your traditional telephones (wired, wireless, answering machines, whatever) and then your telephone acts pretty much as you have used it all of your life. Except that you get every feature you have ever heard of for free (at least from me)... call-back, call waiting, conference calling etc. If you have 10 telephones in the office but only want to pay for 3 "telephone lines" you get a box that is kind of like a router and all of your inter-office calls are taken care of and the incoming and outgoing calls are automatically connected to the "telephone lines" (used to be known as "c.o. line").
When choosing a company look at the total cost ! The big orange box company "tech support" is script readers in India. Is that going to save you money?
Others do NOT include a telephone number in their price. Heck... if I wanted to mislead people while still telling the truth I would tell people my service is free ! (it can be, but I won't recommend it).
Next... TAXES. Some states are still dealing with the matter of taxes. The US congress a few years ago decided it had the power to forbid states, counties, cities etc from taxing the internet. However, states like Florida say that they are taxing the telephone, not the internet. (the internet is merely a substitute for the telephone 'central office'. Florida has a "substitute communications service" tax which means that if you use two tin-cans and a string to communicate, you still have to pay the tax. So if you sign up with a company that is not collecting communications services taxes keep in mind that someday they might go belly-up and your state might come after you for back-taxes.
(Florida has 575 different taxing authorities for communications services, I fill out an 18 page tax return every month.)
Another HUGE consideration for anyone considering VoIP; the International Standards Organization ! Many VoIP services out there do NOT use I.S.O. standards. So if you sign up with them and want to go to another VoIP provider you have to buy all new equipment ! In rare cases, you cannot even change your ISP without a big hassle. If you sign up with a service that is using the I.S.O. standards, you can change your ISP all you want and never even have to notify your VoIP provider (it makes absolutely no difference to me who your ISP is..you can change 10 times a day and I will never care). In a small office or large office this could be a big expense... if you have to buy new "switches" (servers whatever you want to call them) you could have a very expensive bill when it is time to change services. With a company using the I.S.O. standards, you can change VoIP provider simply by entering the new account information into your equipment.
Other advantages are that your VoIP provider should (if you don't choose one of the cheap or orange brands) will give you nearly total control of your telephones. You can see all the calls in and out to each extension. You can restrict all the privileges for each individual. You can control your voicemail. You can control the forwarding etc. Or you can give your employees (family etc) all the control if you wish.
If you have any more questions look at my site
Hunt around the menu and there is a lot of techical information there.