Google Voice is an IP telephony application that allows registered Google Voice website members to link their mobile phones, IP telephony applications and land line phones to a single Google Voice phone number.
Google Voice calls made on smartphones use the phone's carrier network. Calls made or received over the Internet, through Google Voice’s integration with Google Hangouts, use either a data network or Wi-Fi connection. Hangouts is available in Gmail and Google+, as well as a standalone mobile app.
Google Voice can be used to send and receive text messages from the Google Voice number and audio files, texts and voicemail transcriptions can be archived online. Because the linked devices can share a voicemail inbox, messages can be accessed from whichever device is most convenient at the time.