There are a number of Apps that use Google Voice to make and receive calls and text messages, like Groove IP, Talkatone. That means calls and texts in North America are free, you also get your free phone number. Personally I use Talkatone. Bad thing is Google already announced that it will end support for 3rd party apps like these in May 2014, and Google hasn't released it's own Google Voice app capable of making calls.
You can also use Skype, which can do all the calls and texts for a fee. Skype also charges for a phone number so other people can call you.
There are a number of other VOIP services and apps that use them, check out IAX and SIP.