Google Voice can do many things. Not only can it make calls, send/receive texts, and transcribe voicemails, but it can also give you an additional phone number to use for all the aforementioned services. Voice has done a good job of integrating all of this in one package.
Well, Google just released the latest version of the Hangouts App that integrates SMS and MMS. Unfortunately though, Voice numbers don’t work with this integration quite yet (unless you’re on Sprint and have activated your Voice number on your device). If you want to text from Hangouts, you’ll just have to use your real phone number for the time being. According to Google’s Vic Gundotra, “[W]e’re working to support SMS messages for all Google Voice phone numbers by early next year.” Apparently it’s a big process to fully integrate Voice, but Google is on it and expect the melding of Voice and Hangouts in 2014.
Additionally, Google Voice numbers with calling and MMS support for international customers will be made available as well eventually. Carrier issues need to be ironed out still, which will take some time. Third party apps that violate TOS by utilizing Voice for SMS and calling capabilities will also be stopped. Developers of these apps will notified and will be required to halt their unauthorized utilization of Voice by May 15, 2014.
Source: Vic Gundotra G+