Another site to check out is
http://www.psitek.com/ They make a good GSM-PBX interface - with least cost routing setup and the right mobile carrier, you can save thousands of dollars on fixed to mobile calls. I have setup a few of them setup at the Hospital I work for and have saved them at least $20K a year on fixed to mobile calls.
There are other manufaturers as well, you just have to find one that suits your application and a supplier near you.
Just do a search for GSM modems/Interfaces
http://www.hightech-store.com/gsmfctgermany.htm http://www.hightechnologyintl.com/cellinterface.html http://www.nokia.com/downloads/solut..._datasheet.pdf