Actually there are some softphones (this is a software that run as a land phone) that are available in the internet, for example, 3CX or SFLphone, but there is one called Zoiper that is very interesting. Zoiper is a free software that allows you to receive and make voip calls over GSM, 3G or WiFi. This free software doesn’t have advertisements, it has a simple user interface and great audio quality.
This softphone is available for iPhone, Android, WP8, Windows, OSX and Linux. Using a smartphone to use a VoIP service is always an excellent choice! Take a look to this free software!
Free Download: Zoiper