How Google Voice Number Portability Works
Number portability was introduced in the US a few years back. It allows us to switch cell carriers and still keep their previous mobile number. While this was quite a step forward in breaking the carriers stranglehold over telephone users, there's still a little more that can be done. Google Voice for example, has long since allowed users to link their existing phones to their service for free by giving them a "Google Voice number" which then forwards all calls to their other phones.
This isn't a regular VoIP phone service, but it has many benefits since it allows users to have a single number they can give to people using which they can ring all the phones they have access to including land lines. It also makes it easy for a person to switch their mobile phones without caring for what the actual telephone number is since they just have to update their new number in the Google Voice GUI settings screen.
There are many other services which Google Voice provides and is going to provide in the future. However, there was one little problem so far - you couldn't transfer your existing phone numbers to Google Voice.
Number Portability in Google Voice
In early 2011, Google made an announcement that users could finally port their existing mobile numbers onto Google Voice. This meant that users would not have to give up mobile number that they had been using for years in order to make use of its services. There are conditions attached however, and Google warns users that this means that they will incur Early Termination Fees (ETFs) if they're under a contract (which most users are) and that they'll have to get a new service with a carrier to go with their phone.
Once their new service is in place, users can just enter the new number in their Google Voice screen and start receiving calls as if they never changed it in the first place. It means that people can finally and realistically start using their existing number to ring all the phones which they normally have access to.
While Google isn't a carrier yet, there are more and more indications that it will soon become one. My guess is that's when business mobile VoIP will really take off in the US and we'll come to be rid of many of the unfair terms and conditions which we've been laboring under all this time due to the oligopoly of the carriers such as paying for SMS services and contracts.