For a while now, setting up SIP for voice to the outside world through these amazing SRV records has been kinda like being the first person in the world that owned a fax machine: They probably did not get very many faxes, since there weren’t any other machines out there to send them anything.
The idea of communicating to an organization outside of your own is called “federation”, which is defined as the joining of two distinctly different networked systems. The need to connect one domain to another first arose because of the desire to share Instant Messaging services between organizations. From this, we are seeing two divergent philosophies around federation. The first is the idea that not just anyone should be able to federate with my domain. A user in my domain should make a request for the federation, and the systems administrators can work with the systems administrations of the other domain to set the relationship up at the server level. The second is the idea that my domain is open to anyone that wants to send me an instant message, voice, or video call.
The open federation model is more along the same philosophical lines as email and SMTP, but creates the possibilities of voice or video spam, unwanted calls by telemarketing fir...wait a minute, we already have these things on the PSTN! Can you imagine if early within SMTP’s infancy we had adopted the “manual federation” mentality? That protocol would have been worthless if it couldn’t have scaled with the growth of the internet. Sure, we got spam out of it, but there are always going to be people who abuse the system that’s in place. Our world has been transformed by email and instant communication worldwide, the same way that the world was transformed by the telephone back in the late 1800’s. It’s only logical that we take voice communication to the same level that we have email and video conferencing, ubiquitous B2B voice calls over SIP.
Another reason to take B2B SIP voice seriously is that we aren’t going to have the PSTN forever, at least not in its analog and TDM forms. Efforts are underway to establish what the next phase of the PSTN should look like, and by all accounts SIP and direct IP-to-IP communications are the forerunners in the options race. There have been a lot of dates thrown out for this “refreshing” of the PSTN, some as early as 2015, some as late as 10 years from now. One thing's for certain, the carriers will try to push for a solution that is going to maximize their profits and burgeoning monopolies (good article here).
I believe it’s up to IT departments everywhere to move the companies that they support into the new age of voice communications with SIP. This doesn’t necessarily mean ripping and replacing existing infrastructure, as SIP can be added to existing PBXs through IP-to-TDM gateways that do the protocol and medium conversions, all while preserving the dial-plans in place. This can be taken further and setup in your DMZ to enable SIP calls to your organization by using the directory number @ domain name notation. This of course, is a one-way solution, since legacy PBX phones can’t dial SIP URI’s, but it’s a start, and it keeps the phone companies out of at least some of your voice calls. The ideal solution would utilize a modern IP PBX like Cisco’s Unified Communications Manager or Microsoft Lync, both of which can federate with outside organizations. Beware though, Microsoft Lync philosophy to the core is vendor lock in, so the use of standards based protocols are few and far between.
We are at a crucial time for the evolution of communications worldwide. I believe the pieces are there to make rich media voice, video, and text communications operate just like email, in a decentralized, highly effective network of individually controlled servers and appliances. Whatever solution you are considering, make sure that the manufacturer actively supports B2B SIP or at least has it on a near-term roadmap, and operates in the realm of Internet standards. Proprietary solutions may work better in one or two silo areas, but overall, the vendor lock-in will restrict your flexibility and interoperability to the outside world. If you want some help wading through the options available to you, talk to your local trusted VAR that specializes in Unified Communications, or find a good freelance consultant that knows this stuff. Thanks for reading!