On Monday, we were excited to learn that Google Voice numbers were reachable via SIP addresses in the form +1{Google Voice Number}@sip.voice.google.com. As many speculated, this was a strong indication that Google is going to enable “pure” VoIP calling on their system, which is huge news.

However, yesterday morning, we found that we could no longer reach our Google Voice numbers using SIP addresses. Disappointed, we decided to dig a little deeper to find out what’s going on behind the scenes.

Our Investigation

The first thing we did was do an SRV lookup for SIP UDP traffic on the domain sip.voice.google.com. A Service record (SRV record) is a specification of data in the DNS that defines the location, i.e. the hostname and port number, of servers for specified services. What does this mean? In layman’s terms, we performed a query to ask, “Okay, where is Google looking to send the traffic when someone attempts to reach someone at sip.voice.google.com using SIP?”

The query returned five hostnames for inbound SIP traffic. Our impulse reaction went something along these lines: “Wow! Five results?! Google must be preparing for a lot of inbound traffic to sip.voice.google.com.” This makes sense in some ways. If you were expecting a lot of traffic to a new service, you would set up multiple places for that traffic to go for the sake of redundancy.

The next step was identifying the IP addresses of the servers. Despite our efforts (trying the lookups from 3 different locations on the East Coast), we were only able to pinpoint 1 IP address. This means that Google probably only has 1 box right now dedicated to handling inbound SIP traffic to sip.voice.google.com.

So, we see 5 hostnames for inbound SIP traffic, all of which currently point to the same box. Perhaps, then, this isn't the foundation of a giant Google SIP rollout. It’s a little too soon to know for sure. The fact that there are 5 hostnames could just be something that the people in charge of Google DNS did as part of their standard procedure.

But if Google does plan on SIP addresses for all, why this is a big deal?

Now for the fun part. If Google is preparing for a full SIP platform roll out, why is this a communications game changer? What’s the big deal if all Google Voice users get a SIP address?

First off, let’s look at a SIP address. You can think of a SIP address like an email address, except instead of sending emails to and from it, you send voice/video to and from it. Like an email address, A SIP address is comprised of two parts: a username and a domain (e.g. user@domain.com). If two people wanted to have a phone conversation over SIP, they could place a call to a SIP address instead of a phone number.

So let’s imagine that every Google user also had a SIP address - it could even be the same as their email address, e.g. user@gmail.com. As Google now has mobile phones and operating systems (e.g. Nexus One, Droid, and an upcoming Google Voice app in Honeycomb [src]), and a soft phone in GMail, they could be registered with your SIP address instead of a phone number. Then, you could call your friends and colleagues by entering their SIP address, over the Internet, for free.

This may sound a lot like what you have now with Google Voice: You can click to dial a contact within the GMail interface. But, because SIP is an open standard, it’s a lot more than that.

Think about it like the history of AOL: Remember when it seemed everyone was using AOL for IM and email? Then, one day, it burst and people used several instant messengers and other email clients, but could still talk to one another. It’s a lot like that. Anyone with a SIP address could call each other over the internet for free. This is in contrast to proprietary technologies like Skype which are largely closed systems that require workarounds to integrate.

Eventually, if a platform the size of Google moves to SIP, many others will follow. And soon, you may have as much need to dial a phone number as you have to mail a letter to your friend.

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