One phone number to rule them all
Google Voice is a phone and text message forwarding system that lets people use a single phone number to communicate across a variety of phones and devices. The system works by giving you a virtual phone number that isn't tied to a carrier or phone, that when called then rings other phone numbers you specify. Google Voice provides great features like caller lists, custom voicemail greetings, online visual voicemail and of course management of a single number from multiple devices. Google Voice calls can be seamlessly integrated into the dialer of Android devices, while text messages can be forwarded to any phone number or viewed in the Google Voice app or on the web.
Google purchased a service called GrandCentral and launched the service as Google Voice in 2009, offering up an alternative to customers who wanted to keep their phone numbers separate from their phones and carriers. The service was opened for anyone to join in the summer of 2010, and has remained a free service ever since. The main limitation is a geographical one — Google Voice continues to be for U.S. residents only, with no clear indication of an expansion to even the rest of North America. Google Voice numbers will make and receive calls to international numbers, however, with very competitive international calling rates between 1 cent and 10 cents per minute, depending on the location.
While Google Voice is free and has many great features, there have been a few lingering issues that have cast a small shadow over the service. Google Voice is well known to introduce sometimes noticeable latency into calls, as every call is being routed through two phone numbers instead of one. Additionally, the service only supports MMS (picture message) receiving from customers of T-Mobile and Sprint, and doesn't support MMS sending unless you have Sprint carrier integration with your Google Voice number.
In September 2014, Google finally made good on its promise that big changes were coming to Google Voice, including integration with its new chat platform, Hangouts. First the Hangouts app finally started to see proper Google Voice SMS integration, but it turns out that was just the beginning. A new, redesigned Hangouts app is on the way, along with it a new Hangouts Dialer app, which is required for making calls over Google Voice. So, the legacy Google Voice app has been replaced by Hangouts, as well as a new dialer app.