no1daag

I´ve some customers how is contractors, building house, and they are use mobile offices ( small houses). Our servers is win 2000 or 2003 depended on what they using for hardware before. Will be upgraded someday!! My question is if there are some solution for a mobile ( GSM) gateway. Need solution for one line and up to 4-8 lines. They calls for free between each other with mobilephones ( GSM-GSM ) so they want to use the gateway for free calls when the have left the building. Need one cheap solution for one line in a small office (2-4 employee ) . Perhaps using a mobilephone connected to server with cable or bluetooth