How call proxying works with Google Voice

Derek Schauland explains how Google Voice call proxying works, allowing you to receive SMS messages and phone calls from anywhere in the world -- and keep it all online. See what you think of this service.

Last week, I introduced the new version of "Grand Central" — Google's call-forwarding application, Google Voice — and told you about some of its basic features. This time, I want to give you some further details about Google's call-proxying technology and how you can use it to receive SMS messages and phone calls from anywhere in the world and keep all your conversations online. To make this as seamless to the users as possible, Google purchased a block of phone numbers in the state of Montana and the 406 area code. These numbers work only for users of Google Voice.

I'll look at Google's call-proxying system using the 406 numbers and provide an example of why it's such a cool technology.

What is call proxying?

Call proxying works much like proxy servers do for Internet content: the voice provider creates a series of virtual phone numbers that allow messages to be passed around in association with your contacts and their actual phone numbers.

For proxied numbers to work, your contacts must send you an SMS message at your Google Voice phone number. When you receive their message, Google assigns a proxied phone number (or virtual phone number) to the sender and then forwards it to your mobile phone or capable device. When you receive the message, it appears to come from a 406 area code, which is the proxied phone number that Google assigned to that sender. Replying to messages from the proxied phone number allows Google to look up your mobile number and the proxied number to find the contact that sent you the original text message.

An example of Google Voice call proxying

(Note: All phone numbers used in this example will be fictitious phone numbers; any resemblance to an actual proxied phone number by Google Voice is purely coincidental.) Suppose my Google Voice number (GV#) is 123-123-1234 and my friend John sends a text message to it. Google Voice then forwards the text message to my mobile phone, which actually has a number of 122-123-1352. My message from John will appear to come from 406-123-1234, which is the arbitrary number that Google associates with John's text message. When I respond, my mobile phone will send it to 406-123-1234. Google Voice checks my mobile number and associates it with the correct contact and proxy number. Your response appears to be coming from your GV#.

This is similar to making calls from the Web interface for the service (or the previous Grand Central service) where the service would call you first at the number you select and then dial the call for you.

Won't my contacts and other users' contacts get mixed up?

Google Voice assures that contacts belong to the appropriate users based on the GV# and its assigned proxy phone number. Once your contact sends a text message to your GV#, Google appends a virtual or proxied phone number to that contact's record.

For example, as a GV user I am assigned a number, 111-222-3333. When I register my mobile phone as a device within GV, Google Voice can then forward text messages sent to my Google phone number. Google uses the proxy to allow text messages to be forwarded to their users. In turn, this also provides an easy method for Google to ensure that only your contacts receive messages from 111-222-3333.

When you return a call or text to the virtual number, Google uses caller ID to determine the number of the device you are calling from. With the combination of these numbers and checks, Google Voice assures that their proxied numbers will not get mixed up between other users of the service.

See Google Voice Help for more information. Do you see any other potential problems here? What about security vulnerabilities? What's your first take on Google Voice's new features?


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