Here is another link for you to consider: http://www.2n.cz
They make a wide range of GSM gateways. I bought a VoiceBlue GSM box (takes up to 4 cards) from their UK distributor, and it talks to Asterisk over SIP.
ChanGsmBt looks like the solution that I am looking for.
The only problem i have is that our server is tucked out of the way where it wont get stolen or have coffee spilt on it. (more than 100m from the office)
What would be great would be to have a small windows server that connects to your mobile phone via bluetooth and then shares your phone via asterisk.
This means that people in the office could have the small windows server running on their computer, walk into the office with their mobile and easily make cell-cell calls from the phone on their desk.
This would mean I idnt have to build another server for in the office or have to leave my mobile phone miles away from my desk.