The beauty of SIP (Session Initiation Protocol) is that it's a real standard. Unlike earlier IP telephony standards such as H.323 that were more notable in their exceptions and proprietary extensions, SIP devices are designed to work together. Even better, for the most part they do.

For example, Pingtel Corp.'s PBXes are designed to work with any SIP-compliant phone. In fact, the company has spun off its handset division and tells integrators that they can select any SIP-compliant handset that meets the needs of their clients and gives them a good profit margin.

While it pays to do some testing before you leap into a full-scale proposal using phones and a PBX that you haven't tried, you can be fairly certain that putting an SIP phone into a network with an SIP PBX will work.

Still, there are a number of caveats. Some SIP phones may have some advanced features that are only available when used with PBXes from the same manufacturer. However, other SIP devices will usually work just fine, and if you're trying to keep costs under control, you can usually find ways to combine the product mix.

Because SIP is an immature standard there are frequently differences in the way it is implemented. This means that you need to check the details to make sure the product you want to use will do what you want it to do. But that doesn't alter the fact that you at least have the opportunity to integrate a system that does something close to what you have in mind at a competitive cost.

This, of course, is why so many PBX manufacturers are shipping SIP products. They know that this standard is picking up a good head of steam. According to Neal Shact, CEO of < a href="http://www.communitech.com">CommuniTech Inc., an Elk Grove Village, Ill., telephony wholesaler, it's paralleling the growth of the open-source community. And in fact, there's a lot of overlap with open-source PBX products based on SIP already in the market.

Even with all of the caveats, and even with the fact that the standard is sure to grow as it matures, SIP already provides great flexibility for integrators. Even better, an SIP solution, despite its limitations, is much more likely to deal with the future than are VOIP products rooted in the past.

Check out eWEEK.com's VOIP & Telephony Center at http://voip.eweek.com for the latest news, views and analysis on voice over IP and telephony.