It's also worth noting that we found it impossible to transfer a call or do a teleconference with a Microsoft Messenger client. Still, it worked fine for simple phone conversations, though it suffered from more latency and echo than the hardware phones we tested.

Call Me

One area where SIP is making strides is IP telephony. Carriers providing IP Centrex services have long been advocates, and the eponymous BroadSoft SIP platform we used for our testing helps carriers and ISPs offer the service. This cost-effective, reliable turnkey product, assembled from best-of-breed SIP vendors, integrates with its own customizable software to make it easy for a carrier or ISP to roll out IP Centrex and advanced services.

For example, we set up unified-messaging services that automatically sent voicemails to an e-mail address. Web-based provisioning, as well as a Java application called CallPilot, made it a cinch to forward calls to another phone or redirect them to a remote location. We could even conference in other phones when using Microsoft's Messenger client, and we easily set multiple numbers that would ring simultaneously. CallPilot even provided a detailed log of all dialed, missed and received calls.

Need another example of the SIP protocol's flexibility? BroadSoft plans to add IVR and speech-to-text services to its offering by partnering with Holly Australia.

Clearly, SIP is making inroads with carriers and even consumers (see "Start Small"), and though the enterprise voice side is a little slower on the uptake, SIP is gaining momentum there as well. Unlike carriers, many enterprise users have been willing to tolerate VoIP products that have been just as proprietary as the legacy systems they are meant to replace. In fact, some companies are locking into the same vendors that provide their voice and data networks, winding up with deeply proprietary setups.

That's starting to change, though. In our tests of SIP-enabled VoIP phones, we found them not only interoperable, but very functional. In fact, we made a point of using SIP phones exclusively for the duration of testing, and all calls went off without a hitch. Quality was also a pleasant surprise: All these calls took place across the Internet, from our Syracuse University Real-World Labs®, using BroadSoft's proxy and PSTN gateway services at its locations.

Still, despite the fact that SIP connects so well, you will be hard-pressed to find end-to-end SIP connectivity between organizations. The PSTN is still used to talk to the outside world. That means you'll need a gateway translating from SIP to the legacy PSTN technology. The advantage SIP gives you here is that you can choose from lots of gateways based on price and features, instead of being locked into the one provided by your vendor's IP PBX. The gateway just has to be reachable from the inside via IP.

We tested another example of SIP in the enterprise, a SIP-based PBX from Zultys (which also participated in our phones review). The MX 1200 supports as many as 1,200 SIP phones and has all the functionality of a conventional PBX and more, including voicemail, automated attendants and ACD. It also includes a switch, a router and support for standardized Power over Ethernet (IEEE 802.3af). In addition, the MX 1200's built-in DCHP and TFTP servers have GUIs that made deploying SIP phones a breeze.

Market-leading PBX vendor Alcatel also added native SIP functionality to its OmniPCX Enterprise product earlier this year, making it possible for enterprises to choose phones from multiple vendors for use with their PBXs. Alcatel has demonstrated that this vision is for real via a multivendor installation at The Hotel Commonwealth, a Boston luxury hotel: The office phones are from Alcatel, while guest phones are supplied by Pingtel (see "SIP Shows Some Hospitality," for more on the Hotel Commonwealth).

Peter Morrissey is a full-time faculty member of Syracuse University's School of Information Studies, and a contributing editor and columnist for Network Computing. Write to him at ppmorris@syr.edu.

Post a comment or question on this story.