If you are deploying OCS 2007 with the voice technology and mediation server role, here are the gateway vendors you want to be communicating with for a supported gateway. http://www.microsoft.com/uc/pdgtrials/default.mspx

Update November 14, 2007 - This is the Microsoft Unified Communications Open Interoperability Program site so you can see what vendors are certified for what solutions.  http://technet.microsoft.com/en-us/office/bb735838.aspx