This problem can be generated by several things. It is possible that your computer is restricted in using or downloading certain software like Magicjack.
Also, you have to make sure that you have one of the following so the software could run on your machine: Windows XP, Vista, 7 or 8. Also, you got to have one of the following: Broadband, High-Speed Internet, Cable Internet, DSL, WiFi, Wireless, or WiMax and you got to have a present corded telephone or cordless telephone system (you can also use a headset or your computer's microphone and speakers).
For more informations and FAQ you can visit the following official website.