Google Voice is a call forwarding service. Someone calls the GV number, and it forwards it to your actual cellular number. It does have a text service that will act as the inbox for messages sent to the GV number. It doesn't support MMS messages, and will forward them to your gmail account by default. By changing a setting to forward calls to gmail rather than to your mobile number, you can use an app like GrooveIP, Talkatone, or various other ones to make and receiver calls over the internet. This can be done using Wifi, or your mobile data depending on which app and version of the app you have.