FaxCore (News - Alert) has been supporting FoIP since 2005 and TMC has been covering the topic in the TMC Fax-over-IP and FoIP Channels since 2007. 

Enterprises today are more focused than ever on FoIP, but just as in 2007, many “Internet Fax” solutions are now available. However, they’re not all created equal so enterprises need to understand some background about the technology involved, including what it can and cannot do. For example, problems persist over the global carrier network. However, many of the issues have been identified and are being addressed - plus virtually 100 percent of the fax machines in the world are not FoIP capable anyway.   Regardless, the T.38 FoIP standard is very useful within an enterprise network particularly with a virtual OS and VoIP - so pure point-to-point FoIP does not diminish the value of FoIP.   

Many enterprises have made, or are planning to make, the switch to Voice over IP as part of the trend toward convergence of voice and data applications onto a single IP network.  Many of these organizations have also implemented or are considering a Fax-over-IP solution because they can save money by using the same equipment for voice and data, and fax.   That was true in 2006 and is still true in 2012.

As presented at ITEXPO in early 2008, Fax-over-IP also offers plenty of opportunities for resellers (click this link to go back to 2008 channel article): including 1) Growth   2) Convergence (News - Alert) 3) Migration Path 4) Business Continuity 5) Price 6) Simplicity. Fax-over-IP growth continues today at an even faster pace. Convergence remains a major focus of organizations as does business continuity. Also, price advantages and simplicity of operation never go out of style and FoIP continues to offer those features.

While features, benefits and requirements vary among fax solutions, when looking for an answer to your pre-requisites, you should create a shortlist of your requirements in order of priority. While most companies are not running Exchange 2003 or SQL 2005 and have migrated to newer versions, a fax server application that can trace its roots back that far does demonstrate proven experience. 

You will also want to look at which Fax-over-IP solutions have consistently received industry recognition for their products over time.   This can demonstrate that the solution was a leading edge product in 2005 or 2006 and continues to lead the pack in 2012. In horse racing they call it breeding. In the fax server industry, they call it FaxCore.